I don't wanna loose her.. but she doesn't understand...?

You need to work on your fear of losing people before you even start to deal with your relationships. I think that therapy could really help you with that cause it's really hard to do by yourself. You have to find the confidence & strength in yourself to accept those difficult losses that you've experienced & move on in your life. I'm not saying to forget, but to really grieve & face your current relationships without fear. For now, try to talk it out with your cousin & explain what you're going through, and how much that argument worried you. Just be honest with her & tell her how you feel. Good luck ♥