Hung up on a boy I haven’t seen in 3 years

There was this boy who went to school with me when we were about 11-15. We were honestly best friends but he told me many times he had a crush on me but we never did anything about it because we were such good friends. He ended up moving schools due to bullying issues and our contact faded. We didn’t have an argument we just stopped speaking. We’ve talked about 3 times in the past three years but it’s never really lasted long and always fizzled out. I haven’t actually seen him since I was 15 and I am now 18. My issue is I can’t get him out of my head ever. I’m constantly thinking about him, checking up on his social media and now I have absolutely no desire to date anybody else. I might think a boy is good looking in conversation but that’s all that ever happens. I have no desire or motivation to peruse anything with anyone because in my head I constantly think we will end up together. I’ve even told boys I don’t want a second date simply for the reason that I’m so hung up on him. What should I do?