@Egg17 responding to your story because it reminds me a little of ours. My partner's situation was very close to yours when I met him. He'd been divorced for two years and was doing all of his parenting time and visitations at her parent's home instead of his own. He is laid back and took all his cues from her. He was miserable with the situation but he was afraid to enrage her.
The Ex in our case is demanding and difficult. It's "my way or the highway." I never really understood just how hard this can be for a non custodial parent until going through it with my partner. I was raised by a single mom and always saw life from her point of view. I grew up thinking she was a victim and if it weren't for bad ole dad she could have been happy. That thought changed once I got to spend time with my dad as a teen. I've been dealing with my current situation for 4 years now. Here's a few things we've tried along the way. It's not perfect and we may find ourselves going to court despite our best efforts but it's helped with sanity issues at least.
We gave up on reasonable co-parenting recently. Before anyone yells at me for this... we are dealing with a high conflict individual with very little empathy. Every life change seemed to trigger her. She yelled at us when we bought a house without informing her first! We've had public meltdowns, verbal abuse, smear campaigns, and alienation going on. We've chosen to go minimal contact or "gray rock" as it's called if you are trying to work with someone who has a personality disorder. I think this approach works extremely well for people who fall into bad habits and exhibit toxic personality traits. Divorce is stressful and can trigger toxic behavior temporarily. They don't actually have to have a full blown disorder for these to work. You say in your story that you feel like you are "walking on eggshells." There is a great book about Borderline Personality Disorder (BPD) called "Stop Walking on Eggshells" by Randi Kreger & Paul Mason. I'd say don't get too hung up on a diagnosis. You really can't diagnose them and it doesn't matter much. If they are abusive, don't have good boundaries, give you the silent treatment or other manipulative punishment techniques, lie, and are bad mouthing you to anyone that will listen then you need to get healthy and set some boundaries.
Our approach is to put everything important in writing, we don't respond to verbal abuse at all, and we don't respond to demands or yelling. We stated what our boundary is and we stick to it. If she calls on the phone and she starts yelling we hang up after giving one warning. She knows this and her behavior did a 180 degree turn around for over 70% of the interactions that are now required. This is hard to do because you are very tempted to defend yourself every time you get someone screaming at you and calling you names. Just don't. It won't work and it serves to escalate the situation. Correct factual information only but ignore the name calling and judgement. With a little practice this approach does work. It took many years to get to this point. We started out with the co-parent paradigm and as it became crystal clear this individual had little to no boundaries we moved to a parallel parenting approach. We no longer get those midnight calls insisting we co-sleep with the child. The public tirades at exchanges have all but disappeared. The demands to do things her way are a lot more measured and thoughtful. Like I said it's not perfect but it gave us some sanity back. It's a process. It's very likely your Ex will not change or get better if you are dealing with something like BPD but you can learn to manage it.
You will also not want to depend on her for important information. We discovered when we just trusted everything she told us over the first few years lots of facts were left out or she would spin the situation in a way that wasn't entirely honest. Now we speak directly to the schools and to the doctors for all situations regarding the kids. If we need a diagnosis we get one ourselves. The Decree allows us to make those kind of evaluations as long as it is not medically intrusive. Make sure yours does before doing something like that. Having our own parent/teacher conferences had a nice benefit of making her more accountable. She was ignoring a very important issue in our case. The youngest was in danger of repeating first grade!
The next thing is to keep in mind that your Ex is a human being. Don't be cruel and use some of the strategies I mentioned to push her/his buttons. Your goal in dealing with your Ex is to minimize the conflict you must deal with. Edit your emails to strip out anything judgmental. Avoid trying to resolve issues verbally if possible. People use their problem solving brain a lot more when they write vs. when they speak. If she or he is someone with a true personality disorder they really are suffering beyond anything we can comprehend. It's okay to protect your sanity but any attempts to 'school' your Ex on how they should act will backfire. Also never tell anyone you think they have a mental illness. This will also not go down the way you think it will. You'll just end up becoming a target for that person for much longer than necessary. If you get a new SO in your life it's best to minimize their involvement with dealing with a high conflict Ex. It's not fair but low conflict is the name of the game. You moving on often times triggers feelings of abandonment which will ultimately translate into bad behavior.
Read your Decree and know what your rights and responsibilities are. This may sound obvious but we were surprised by something that came up in the Decree 5 years after it was drafted. The wording is thick with legalese and difficult to understand so it's easy to miss something. Use a lawyer to help you if you need to. Once you understand your document this becomes your bible for how to respond to an Ex that doesn't respect you or your boundaries. Everything in that document is meant to provide a way to resolve parenting issues when you can't agree. Use it. She is in violation if she is not releasing the child to you for any reason at all - extracurricular activities or anything else - during your parenting time. Document these infractions and when you've got enough documentation and funds go back to court if you have to.
Let go of the notion of how she should be spending your child support dollars. Our Ex hasn't worked in over 10 years and has no intention of getting a job. Periodically she asks for more money even though she is getting more than the maximum. He gives her a percentage of bonuses even though that is not required. In our case she lives with her parents and most of her living expenses are paid for by them. She uses child support money for an extremely expensive vehicle, travel, and smoking. It's aggravating that the money doesn't go directly to the kids but there is very little that can be done about that legally in our state. It's best just to let that one go.
Lastly keep your eye on the prize. Raising terrific kids and having a good relationship with them with the least amount of conflict is a good goal to strive for. You have to model flexible thinking and good behavior for your kids because they aren't getting that with your Ex very likely. I know that sounds trite but the normal day to day things other parents take for granted like raising your voice to your kids or even spanking them aren't a luxury you can afford. You may have to learn really advanced people skills to use with the kids. Do some reading or get counseling on how to improve your relationship with your child. If your kid is being fed criticism about you don't panic. Our child repeats everything her mother says about us. When she was 5 years old she and I use to have conversations about mommy believing we were #$!% (fill in the blank). I would ask her what she thought and she generally replied that she did not agree with that. I would tell her that one day when she grows up she would be able to think about that more deeply and that it is okay to trust her gut instinct.
All the best to you and hang in there!