I am falling for my best friend and it is ruining my life

We have been friends for six years. He has seen me through two breakups. I have been at every family thing, know his mother by name, have a drawer at his flat. Somewhere in the last year something shifted and now I think about him constantly. I have not said anything because I am terrified of losing the friendship. But pretending I feel nothing when I am around him is the hardest thing I do every day. Has anyone navigated this? Did it work out or did you lose the friendship?