I slept with my friends boyfriend.. Should I tell her?! Help!?

It happened one night back in November we were both a little drunk however... I have really fallen for this guy. They have been together about 3 years and moved in together just after all this had happened. I work with this guy and his girlfriend works for the same company different days to us. I wouldn't want to hurt her but can't stand her not knowing what he is really like and also everyone thinking he is the perfect gent. I know the blame will mainly come on me but I can't stop thining about it all I really like him and think that if my friend knows it will be the only way something can get sorted... Should I tell her or hope I gradually get over all this?