Anxiety about seeing Ex

I was in a relationship with someone for a year and a half. During this time he (my ex) was struggling with very severe OCD, anxiety and family issues and I was separately dealing with my own mental health issues as well. During our relationship, he would often tell me his worries and go into the details of his horrible (sometimes suicidal) mental state. I don’t think he did this to purposely cause me stress, he just needed someone to talk to during that super awful time in his life. However, over the course of the relationship I developed very severe anxiety issues with him being my biggest trigger. Towards the end of our relationship it got to the point where I would dread talking to him or texting him because I was so worried about what I might hear. Being so in love with someone and caring about them so much only to hear on a regular basis that they felt so low was devastating for a 16 year old (me then) to go through. Long story short, that relationship was traumatic for me. We broke up about 9 months ago with the intention of staying friends and still texted each other fairly regularly after the breakup. We broke up mid-pandemic and when the time came for school to start again he chose to go back in person and I chose to be online so we didn’t see each other at all after the break up. Until, a school obligation 2 weeks after the breakup when he wouldn’t look at me, talk to me, or even be vaguely around me. At the time I was shocked and hurt since we were able to communicate fine over text and what I should’ve done was made an effort then to clear the air or sent a message afterwards asking what was wrong but I didn’t. Instead after that day our contact with each other faded into nothing. We had to see each other once a week for months because of rehearsals for school (orchestra) and each time it was as if we’d never met. Worse actually. It was and is unbearably awkward between us. As I said before, when we were actively dating he was the biggest anxiety trigger for me but once we were apart it became so much worse. I’d get panic attacks every time I had to go to rehearsal just because the thought of seeing him with all of that tension and awkwardness was too much for me to handle. Now, the reason I’m writing this is because school is starting again and I’ll have to see him in person everyday. I’m freaking out. I’d like to be able to talk to him at least casually again because while we haven’t talked in nearly 9 months I do still care about him as a person but at the same time, the thought of talking to him at all makes my stomach churn. I’m not sure what to do because it feels inappropriate to text him randomly after 9 months of nothing dragging old things back into the spotlight in an effort to fix things. It’s just so awkward and I hate that he has so much power over me without even meaning to. Please help. It’s my senior year and I don’t want it to be tarnished with anxiety over a relationship that’s long ended.