He did it with my best friend.

me and my boyfriend were dating for almost 10 months, it seems like such a short time compared to some stories on here, but never the less my feelings for him were unconditional.
just recently i discovered that he'd been sleeping with my so called best mate, behind my back. i have no idea how long for?? or why?? i knew me and my boyfriend were having a few problems but none severe enough to excuse this!!
i feel sick, worthless and well and truely abandonned! moving on from a cheating partner, i believe, is one of the hardest things anyone can do!!