it sounds to me like the problem is in your guitar...what guitar are you playing? what wood is it made of? what is the neck joint like? (set or bolt-on) and how many frets does it have?what strings are you playing? all of these factors influence the sustain to a greater or lesser extent.

A good setup could fix the problem (especially if it only started now)...maybe a even a re-string could solve it...if needs be, get a compressor...no guitarist should be without a compressor