You sound JUST like me 10 years ago.
We now have kids, one with quite severe special needs and are separated. Honestly I could never have forseen that we ever qould have broken up, we were sooooo in love, just perfect together. I love my kids but parenthood definitely ended our relationship.
My thoughts in no particular order are:
Many people love to tell women who don't have children how those women will feel 'when' they become mothers. What I realised after is those people were just talking about how they felt when they became mothers. Your best sense of how you will feel if you became a mother is yourself. My sense before I had kids was that I wouldn't much like being a mum. The things I enjoy are a combination of super-quiet solitary things like reading books, or super-wild partying and travelling. Neither are compatible with parenthood. And guess what? I had kids, not a personality transplant. I still enjoy the same things I used to, I just don't get to do them any more. I still find babies and toddlers really boring.
Mine are a bit older now and great fun and whilst I don't much like motherhood per se, I do love my kids as humans in their own right and love being with them now. However if I had my time again I would say a different life path which didn't involve parenthood would have been a happier one for me, and I know now that I was best placed to judge for myself how I would feel about parenthood rather than listening to other people tell me how I'd feel.
Maybe I'll be hung out to dry for this comment but I'm just using an anonymous internet forum to be totally totally honest. I am a very good mother and very dedicated and if you knew me you would not guess any of this to be true.
As for our relationship, the fact we'd been together so long (also 14 years-ish I think when we had our first) worked against us, because we were so used to our patterns together, the things we did together (lots of dates, travel and spontaneity) and being able to put each other first, the relationship just could not adapt to having to put another person first. Also stuff like when it was just the 2 of us I could do all the housework and cooking and also work full time and I didn't really mind, I couldn't do any more when I was also looking after a baby and he felt SO HARD DONE BY that he had to start doing his share. Again because his whole adult life he'd been used to one way of doing things and it was just a huge change for him to process.
We have discussed openly in the subsequent years in therapy and so on. He now accepts that his sudden desperation for a baby was about wanting external validation on what a perfect life 'should' look like rather than an actual understanding of what parenthood would actually entail and wanting the real-life warts and all version. He also says with the benefit of hindsight he too would have accepted my view and we wouldn't have had kids.
We were also some of the first in our group of mates to have kids. After the first raft of us had babies a few mates decided to stay child-free by choice and they are super happy fulfilled people. None of them regret their choice.