Im a girl and i have fallen i love with my guy best friend?

I really would suggest that you just tell him. It isn't your fault that you panicked, but he might take it the wrong way since he confessed 2 months ago & you're only sure of your feelings for him all this time later. Remember though that telling him won't guarantee to make him stop seeing this other girl. Tell him, talk it out with him, and then see what happens from there.