I have 2 PSTs and actually carry them more than my flap bags. I had that problem of the sholder straps keep falling off. I twist the strap so one is on top of the other one and it helps. Or sometime I even carry it in my arm.
If you're not in love with it, sell it even if you'll get less than what you paid for. At least you're making some money than it is sitting in your closet. Then use the money to buy something else you really love and will use.
p.s. one comment about returning your bag at retail. It's not right to return the bag if you already used it even once. No one wants to pay the insane retail price and end up with a used bag. Just my honest opinion. GL with watever you decide.
It's just not a comfy bag. Don't worry. I didn't intend to return the bag. It was just something someone suggested in this thread.