Eleanor Roosevelt once said, "No one can make you feel inferior without your consent." That is to say only you can trap yourself in this overwhelming cocoon of regret and embarrassment. Some people cause pain and some people allow themselves to feel this pain. People are people and honestly should always be treated as such. It is important when addressing the situation that you don't objectify this person and realize that they hope, hurt, fall, and triumph just as you or I but something is causing them to act out in a way that is very hurtful.
That being said, you should not beat yourself up for bringing this person back into your life. Our parents and guardians always raised us to expect the best in people, find the best in people, and hold on to the best in people, but they don't always tell us that sometimes that little bit we hold on to can blind us from the bit that packs a powerful and painful punch. You can only be embarrassed if you are ashamed of your actions, which you shouldn't be. This person that is hurting you should be ashamed for acting as they have when given an opportunity of a life time. Anyone that is willing to look past whatever caused someone to be put out of their life to allow a second chance is a person too kind to be put through it again. It is in our nature to hope and seek the best in terms of change in others but sometimes it takes longer than we expect.
To address your feeling of regret, just don't. Regret is a horrible emotion, almost as debilitating as guilt. Everything happens to teach us something. What that "something" is depends on the situation and the person walking the path. Regret is something we feel towards mistakes, it's generally a "fight or flight" emotive response but nothing is a mistake but instead experience. A lesson perhaps. Don't give this individual the satisfaction of folding in upon yourself. You were a very big person to be open to the possibility of their change. You should be proud. They blew it. They took your caring and generous gesture for granted. Perhaps this is just life's way of showing you that you can only go so far for others before they must start to walk to meet you.
The most important thing in all of this is to cut this person off, for now. Never let them return to you as intimately as they were and tell them that you are there for them when they are ready to meet you half way but you have to be just a bit selfish this once. It's hard to let someone go, I know. In the long run you are doing not only yourself a favor but this other person too. Keep this person at a distance for a while, let yourself feel the pain and conquer it in your own time. When you are ready, and in an emotional place where you can come to terms with why you two are not the greatest match, then let this person slowly come into your life. Be there for them, maybe they will be there for you, but don't relive the past. Push forward and move on.
I said a lot but I just want to help. I wish someone would have told me this but I had to figure it out through two painful years of being too generous and open. I let someone walk all over me before I finally went to my mom and she said sometimes our best cards are those not played. Sometimes you have to walk away. Sometimes you have to be selfish is what I guess I'm saying. I hope I helped even a little.
