Dr Jekyll and Mr Hyde springs to mind. That's what I used to think about my ex.
On one level he seemed so nice, almost too nice. He used to make me a lovely packed lunch for work every day which my colleagues thought was amazing. We spent ages decorating our house and making it really nice so it felt like a home because he knew that was important for me. We went on my dream honeymoon to the Seychelles.
But it was as if he was being too nice to compensate for his angry outbursts and occasional psychotic behaviour. I eventually realised he was saying the things he thought I wanted to hear and doing the things he thought I wanted.
But by then it was too late. We'd already had one child and I was pregnant with the 2nd and his true self really came out. Probably the pressure of too much to do, he couldn't maintain the 'nice' mask anymore. What had been occasional outbursts became regular provocation and sarcasm, along with drinking in secret. He still tried to be 'nice' sometimes, but the anger and resentment inside him bubbled through regularly.
In the end, I couldn't take any more, I left when the kids were 4 and 7. They are now 16 and 19 and the ex and I no longer speak. I suffered years of abusive phone calls and emails from after our divorce, he never forgave me for leaving him. When my youngest reached 16 I decided to end all communication as any attempts to co-parent with him in an amicable way were impossible.
Thankfully I am now married to a lovely man. He is not nice sometimes and horrible sometimes, he is always nice and that comes from the heart. It's not something he has to try and do, he is just himself. He would never make me a packed lunch for work but I don't need that anyway, I'm quite capable of doing my own. What I wanted was a relationship with mutual respect and care for the other person and the relationship. And I found it and am very happy and feel sure this is forever.
I don't think your partner will change unless he has a huge amount of therapy over many years. I understand from my kids that my ex is having counselling at the moment as he is still experiencing big problems in his life, 10 years after our divorce. I just hope one day he sorts himself out, but I am just so relieved I don't need to be on the receiving end of his abuse until he gets there.
It might feel like a big upheaval selling your house together now and admitting failure. But believe me, it will be much harder further down the line. I started to see hints of the issues with my ex after we had bought a house together before marriage and children. But I didn't really want to admit it to myself and everyone else at the time. Because he was mostly nice I decided to stay and give it a go. I was about the same age as you and was worried if I left I might not meet anyone else in time to have children as I felt the biological clock was ticking.
In your shoes I would get out now. Seriously. Or see if you can get some counselling or therapy so you can discuss how you are feeling with someone and get some support with your decision.