I'm no expert but if the area is made up of the same type of breast tissue as females then once sagging occurs, there's little that can be done to make them firm. Biulding up the underlying pecs can fill the area out. You say you did 'lots' of pec exercises ... how much (and for how long) is 'lots'? It takes a pretty long time to build an appreciable amount of muscle.
I suppose Lipo could be an option although i don't know a lot about it (maybe there's someone more knowledgeable than me who can advise you).