I rejected him, why can't we still be best friends?

OK so my best friend that I have known forever and liked forever (and he has liked me forever) asked me out and I have always dreamed of him doing that but when he asked me out I started getting thoughts of what we could lose if we did go out and so I said no. Now the problem is in the past year he found a new girlfriend then they broke up and I can still tell he likes me and I still like him but he hasn't spoken to me since he asked me out and I miss him! What should I do? Should I start to talk to him? Should I ignore him? I'm to scared to ask him out again so what should I do?
I rejected him, why can't we still be best friends?
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