When I'm shooting my AR15 usually my left shoulder gets tired from holding the gun up via the hand guard. I lift weights, but imagine it takes some of the smaller muscles to support the gun.  Anyone else have similar problems?  What is the best way to get rid of this shoulder fatigue?  Thanks for the help guys I appreciate it.

1. Get a sling if you don't have one. It will help greatly with muscle fatigue.
2. Possibly change your stance/grip.  If you are holding the rifle with your left thumb on top the barrel ( left arm chicken wing ), try sucking in your left elbow to the weapon, and move your hand so that the rifle lays in your palm. Bone does not fatigue, so get as much of the weight supported by your structure as possible.
    If your stance is squared up to the target, your arms are naturally going to be farther out from your body, and thus the muscles will fatigue more quickly. Try standing in a natural boxing stance. You'll most likely be able to rest your left arm against your chest/body armor this way, reducing muscle fatigue. Squaring up to the target has become popular due to the military's use of body armor, but I personally don't like it.  It puts you off balance, and is uncomfortable ( ymmv ). I'm used to the interview stance ( boxing stance ) from working in LE, so that is what I prefer. You can interview someone, fight, or shoot, all without changing your stance. You are also less likely to be knocked backwards, and helps with recoil. The only downside is that your body armor isn't perfectly square to the target, but I feel the benefits greatly outweigh the risk.
Again, ymmv, but these have worked for me.
 
 

Agree. Try changing your grip & where you grip the handguard. You also may have the stock to far extended & messing with the weapon balance.