My 2c and apologies if some of this is a bit harsh, so try focus on the message:
I haven't been married, and I don't see myself getting married unless it offers both parties a benefit like emigration purposes, etc.
But I have been in 4 long term relationships so far (obv. there have been short things in between)
1x 4 years, 1x 2 years, 1x 3 years, and my current which has been going for more than a year and a half. I lived together with people in the two long relationships (and current because of lockdown); so in all aspects it is like being married except the legal schlep.
Ok so now for my advice:
When my first relationship failed, I was devastated. I cried at night, couldn't sleep, thought I would be single forever and culminated in checking myself into a hospital. Mind you; I had just moved cities and started a new job, so maybe a lot of stress at the same time. Still it was no picnic.
As soon as I recovered from that (few months later), I was scared I would be single so jumped into dating immediately and wow the difference from before when I didn't know what I was doing. I had no problem getting dates or keeping a lady interested.
It really is much easier to start over, the baggage you need to fix is just not worth it. Every single relationship I got better at choosing someone I got along with better.
After that first epic fail, I also handled my next breakup much better. Like my first relationship I NEEDED, my relationships since, I'm in because I enjoy it and we make it work. When it stops working, then we part ways.
Lastly, a lot of people think they have a LOT in common with their current partner, so even though there is a lot of pain in the relationship they keep at it. There is a point where there is too much pain and you have to take what you learned and move on to something new.
Cheating for sure is one of the things that is just not worth trying to fix, in my opinion. Especially for someone who has such weak reasons. I mean next time will she cheat on you because she wants some ice cream and you didn't want to buy her some?