Is anyone else having trouble with the zippers, especially the main one? It seems awfully hard to pull it open. I don't want to pull it too hard as it might break. Also, I can't figure out where to strap the shoulder straps on. I just got it and it's going right back. It's also very big. Too big to wear on a shoulder, too, I think, as I can't put the straps on to try it out.