Can I Sue Ex Bf For Going Back On His Word?

Location: Illinois, United States

Throwaway because he's on Reddit a lot.

BF and I were dating for 4 years and we moved states a few months ago. Part of the deal of the move is that he would support me while I finished my nursing degree because I didn't want to move. He promised me I could be on his health insurance after the new year if we signed affidavit saying we were domestic partners. On it, it says it's a legally binding document and that we both agree we are in an exclusive, committed relationship and have lived together at least 12 months. We signed it, but he never actually put me on his health insurance.

Well, he cheated on my right before new year's and left me a few days later. (We were in an open relationship but agreed we were going to get married. Instead, he made plans to leave me for her and then did. So I think it's cheating but it's complicated.) I have the affidavit and a few other things (texts saying I could be on his health insurance as long as I needed, stay in the house as long as I needed, and that he would keep supporting me financially until the end of May.) I never insisted he put my name on the house (I didn't think I needed to) and now this has happened and he has slowly changed his mind about everything over two weeks. No longer wants me to stay, no health insurance, starting to walk back any financial support.

Does any of this give me the right to sue him? Especially since I have some things in writing, even if they're just texts? He's been pretty awful about all this and I'm hoping that there's Something I can do to not be fully screwed :/