I doubt a Flash 50 is designed for carrying that much weight. That is issue No. 1. It may not fit you right, which is issue No. 2. I am fairly small and 50 lbs is too much for me to carry very far. If you want to carry that much weight, try a different pack, like an Osprey Aether. I've tried those on and they fit me pretty well, but the caveat is that I've only done so in a store, not on the trail. My pack is a Kelty. Make sure your belt is wide and stiff enough to carry that weight on your hips, rather than your shoulders. With my pack, I try to load and fit it so that my hips carry most of the weight. Give that a try. The other solution is to lighten your gear, but that can require a fair amount of rethinking and re-equipping yourself.

If you get a new pack, don't overdo it. Bigger isn't necessarily better. I made the mistake of getting an Arctery'x Bora 80. It was too heavy, too stiff and I sold it after one trip.