I am sorry for your story - being cheated on isn't right and I can understand how it feels when you gave so much.
My story is almost like yours but in reverse. My ex wife cheated on me - one day I was eating toast in the morning and she just ended it. No warning - just it's over, I've found someone else and you need to be out by the end of the week. I was in my 30's and had to start again.
I knew something hadn't been right for years as she never wanted to commit to purchasing a house (even though we both at the time had good salaries).
I found out that she had been sleeping with a so called family friend for over a year.
We had a difficult divorce, she ended the marriage but made it so hard to actually get a divorce (delayed getting paperwork back, quibbling over small things that in all honesty I could not of cared less about).
In the years that followed she managed to lose her job (she claimed that she had a break down and the company made it difficult for her to return, I have since found out that the "breakdown" was actually brought on because they were moving to terminate her for misconduct).
She had to take much lower paid work and has constantly been after more and more money from me. I have always paid well above what the CMA determined was appropriate (I was quite happy paying over 1K a month in Child Maintenance).
I on the other hand have rebuilt my life and done well in my job and have a stable and healthy income. I don't have a partner - but have had relationships and overall I have been happy.
Over the last few years her behaviour has become more and more erratic - where it is clear that she is quite bitter that she perceives that my life is better than hers (mainly financially as that seems to be her primary agitator). It reached a point last year where our child elected to come and live with me as they could not take her mania any longer.
So, I am a full time dad as well as working full time too. However she still contacts me randomly demanding child maintenance money for a child who no longer lives with her and hasn't done so for over a year.
She has demanded that I pay her rent when her wages and universal credit don't stretch far enough (not sure how that works).
She has told our child frequently that it's not fair that I have what I have earned and that I didn't give more and that I "live the life of a king" - when the truth was I always stepped up to provide for my child.
When she had mishaps in her home with appliances I would replace them as she could not afford to as it was for the benefit of our kid.
I struggle with the bitterness and contempt that she has for me and, what is constant harassing behaviour - as I just don't understand why she can't move on and let it go of a relationship that ended over 17 years ago now.
I can't help that things have by and large gone well for me - I, like her had challenges and overcame them. I accepted that what had happened was the way things were and it was down to me to control them.
During the times over the last 17 years where she was doing well and things weren't quite working out for me - I never gave a second thought to how fair that was as I was always grateful that I was out of a toxic relationship with someone who just didn't love me and was willing to betray me.
I know that it is easy to say to someone "oh, just move on" - but - in all honesty if you can find a way to reconcile it in your head you'll be much happier.
There's obviously much more to what happened in my story - but, I doubt that anyone would want me to go on all day about it.
I wish you well.