I really can't believe that I'm still in love with my high school boyfriend
Well are you sure you are? What makes you think you're still in love with him? Is there a possible future in it?
We're practically best friends now, but we met almost every day and we shared some good times.
Seeing an ex boyfriend can bring back a rush of emotions from your past, but you shouldn't dwell on those emotions too much because this can be unhealthy. If you're stuck in the past you'll have difficulty moving forward and meeting someone new.
It's okay I've been holding it back for so long.
At this point maybe you need some distance from him, so that you can move on. Holding onto this old flame will only stop you from having a healthy relationship in the future.