I have no idea what to do

Last year I went through a really rough time with my mental health/suicidal thoughts/anxiety and insecurity which affected me nearly everyday. I also normally find it really hard to talk to people and make friends and obviously my bad mental health made things only worse.
I told my flatmate that I liked him even though I knew I had already made things so bad and so I told him because I thought things couldn’t get worse. He told me he would prefer to be friends because we’re flatmates and it was basically left at that when we both had to go home early because of the virus.
However I did get the sense he liked me, or found me attractive I guess, and to cut a long story short, I messaged him online recently basically due to him prompting me to (long story). He never opened the message but never blocked me so I just assumed he didn’t want to talk anymore and I left it at that.
However, recently I have become aware (again long story short) that he wants to talk to me but he doesn’t understand why I am so awkward and nervous all the time and he doesn’t know how to talk to me. I don’t know whether I should message him again and what I should even tell him. I want to be honest with him but I don’t know how to. Honestly I was shocked that he was even still thinking about me, because we were never really even friends. Any advice? Don’t want to come off as too desperate but also life’s too short to not go for it? I don’t know what to do.