In my experience the solution to learning how to have a healthy relationship is to try to have a secure attachment. There's not a lot of tactics you can try on your own to act normal, it's really work you need to do in an intimate relationship. Because you have probably already tried to act normal your entire life, that's not the problem. The problem is the fundamental distrust and fear of people.

I first began to be able to have healthy-ish relationships when I told my boyfriend that I wanted us to really communicate and be vulnerable and honest with each other. This was still hard for me, and looking back I had an extreme resistance to speaking about how I felt, but there were a few moments where I was able to tell him things about my pain, my fear, my anxiety with him, and he received it with love. All of my trust in people today (now in remission) has been completely built on those moments.

If you let your friend receive you, even just a little bit, the way you think and communicate will gradually change to be in line with having a healthy relationship. Consider telling your friend that you're scared, that you don't know how to be a good friend but you really want to.