8 Years, Time to Bail?

Hi I’m looking for advice if I could, my head is a mess & need to download so excuse me if this doesn’t all make sense.
Although not married my girlfriend and I have been together for 8 years.
2 years in she lost her brother suddenly whom she was very close, this opened a dialog, as macabre as it sounds, about her death and what I would need to do, in detail, should it occur.
I extrapolated this to its end - on my own, holidays for 1 etc etc & quite liked what I saw - maybe this sowed the doubt…
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We have a decent age gap & with the clock ticking she wants kids - I don’t - I just don’t seem to have that part of me installed. I believe she has not (or does not want to) picked up on the fairly obvious signs I don’t them.
Talking with her she has now decided she’s doesn’t want kids either and would give them up for me.
I think this is unfair on her & if we stayed like that she would hold it over me for the rest of my life. I know she would be a great mum.
I think I could live with hitting old age and regretting not having kids but could I live with round the other way? Am I just scared?
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She is not the sort of person to just go out & find someone else. (that would probably make this whole mess easier) She’s very shy & not very confident, where as I more of the opposite.
She’s is also utterly faithful - you couldn’t ask for more. She’s not very girly (on the one hand I like this, but sometimes I wish she was more…)
This is going to sound big headed or egotistic (_I don’t mean it to be but I’m not smart enough to put it any other way) but she tells me she’s world & I know if I left it would devastate her.
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Throw into the mix another woman whom I am attracted to.We work together & she’s lovely, she’s almost the opposite of my girlfriend - aaaand they are best friends!
My girlfriend knows there is an attraction there. It turns out we also share more in common than me and my girlfriend and the age gap is much much closer.
We get on like a house on fire and I genuinely could see us as a item.

So all of this is just swirling around my head.
My girlfriend knows there’s a problem in our relationship & is desperate for me to open up, but I just can’t. I can’t - it would destroy her.
To try & work out if this other woman thing is just my ego running wild & I’m actually double parked in the friend zone, subtle enquiries were made & rumour is she does dig me but I know she’s no home wrecker. This hasn’t helped.
The more I try to put her out of my head & concentrate on what I have to loose - the more she creeps back in.

So you need to ask yourself, ‘Why am I stringing her along if I’m not totally into her and should I devastate her this way instead of cutting her loose and let her possibly find the one for her?’ Pretty selfish, bro.