I need some advice here please!!

My x girlfriend and I was together for about 6 years, around the time I decided to leave her son was at least 17years old in college and her daughter was 11 or 12. My X girlfriend and I had a great relationship and had chemistry out of this world. For the first few years it didn't start off to bad with the kids. I would say things start going for the worse as they got older, the daughter never listened to me, my biggest problem was the disrespect from her son who tried to fight me, threatened me with a knife and thru it all I was still there showing love and trying to be the best stepfather as possible. Her son started college at 17years old and was very proud of him starting at a young age. When he began college he didn't have a place to stay and wanted to stay on campus, but the only thing was her mom didn't have credit and I did, so he asked if I would cosign for a apartment for him on campus. I told him I would, apart of me was saying don't do it. But I did it anyway. Over the next few months while he was in college, my X GF daughter started to act out, she really didn't communicate with me at all, she walked around the house ignoring me and only dealt with her mother. I did my best to try and reach out but nothing in return. There was times where her son would call and ask for money which we always sent and then it gotten to a point where he started stealing money from him mother to pay for school and other things he needed. Well a few months later I started getting calls from the apartments that he was living on campus saying that he was months behind on rent. We would send money to cover his rent because we wasn't sure what he was doing with the money he had alloted to pay his bills. Know at this point with work, and paying all the bills I started getting stressed out and then shortly after I get another call from the apartments he was living at on campus that he abandoned the apartments and trashed them leaving it filthy and leaving me with a balance of $5,000.00 in damages and back rent. Man I was heated, when I talked to him he said "It's just credit" and it's no big deal. I said it's no big deal to you but it's very serious to me because I have to resolve this issue. At this point I don't know what to do, I was upset at my GF & her son. A month or so later I told me GF that I needed time to try and resolve this issue because I could get sued, so my decision was for me to find my GF a apartment that she could afford and I would go stay with a friend and try and save money and resolve this matter. Well sadly we never moved back togethe but stayed in communication. While she had her own apartment I didn't go see her as much as I should because I felt betrayed, I wasn't able to get a place in my name and it took me 2 years to bounce back and able to get my own place. I felt really bad that she fell ill in her own apartment and I wasn't there. Till this day she is still battling with her terminal condition that she monitors. She tells me I threw her away and it wasn't even like that at all. I told her that it would be easier for her to get a place of her own that she can afford and allow me to save money to clear this major debt that her son created. Well it didn't work out like that. During our entire time we was together I supported her in every way I could. My two kids would come over time to time, but really didn't want to deal with the disrespect that I was going thru with her kids. 

Well 2 years later I ended up meeting a wonderful woman that has lifted my spirits and been such a light in my life, she has no kids and she's WONDERFUL with my two teenagers and my teenagers love her and I love her deeply. My x GF and I are friends still but LOVE is still there and I'm torn that I made a wrong decision by walking away. She keeps telling me I threw her away, I don't know if it's healthy to be friends with her because how we feel about each other still. Her son is a grown man know which is 21 and still hasn't reached out to me to apologize or even offer to resolve the matter he caused. My emotions are everywhere and I need to see a therapist to talk about this because I feel that I am holding all these emotions in and it's not healthy. I feel bad that I left the way that I did, but I was pushed with my back against the wall and betrayed, I feel that my GF knew about what her son was doing but never tried to step in. I do miss her and wonder what IF i would have hung in there and where would we be know if I would have stayed. But I have such a wonderful woman now who's been down with me and I her. Love, Chemistry & Connections is a crazy thing... Any advice would help.