I can truly relate to your feelings. I am dealing with the same thoughts and hurts. I have supported and encouraged so many things with little or no thankfulness. Put her through by letting her stop withing and cheered her on while she went to nursing school and then supported her while she decided she dosent want to be a nurse and go to college for a psychology degree. Then she finally graduates snd I was so excited to finally spend time with her and she now wants a divorce. I go through these thoughts so many times as to why I am not goo enough or what's wrong with me that I can't make hr happy. To the point of self distruction. If its one thing I would like to say in terms of encouragement it's this... you may have your faults but you arnt to blame . It takes two and you obviously care. Its their loss if they can't be good enough to see what they had. And most times how lucky they were. Reach out to your church or friends . I bet they could see the truth and your sacrifices

It is hard to understand how some people can just walk away. In my experience, people don't just walk away. Either they've been telling their partner what was wrong and the partner ignored it or was unable to do anything about it and finally they had enough and left. I've experienced this where they ignored it and then being left was a huge surprise and they couldn't understand what went wrong.
The other (and in my opinion more common factor) is it has nothing to do with you, it's something inside of them that makes them unable to stay. Trying to understand it will just make you crazy because you don't really know what they were thinking or feeling. I find that very hard to cope with. I also struggle with not internalizing it (why wasn't I good enough, etc.) Logically I know I was more than good enough but emotionally that's harder to accept. Wish there was a simple, easy answer but there isn't. I also think that because we aren't made that way, it's really difficult for us to understand that behavior.
After my ex left he told me how guilty it made him feel when I would do things for him. He did nice things for me ALL the time and was so good to me but he didn't know how to accept that treatment in return. He told me I was the first person he'd even been with that didn't want something from him. He had no idea how to deal with that and ran from it. I know his leaving had nothing to do with me but it really doesn't make it any easier.
All I can say is hang in there. It does get easier to cope with.