I'm kinda stuck in a rut in terms of practicing, I've been out of practice for about a year now, though I've been playing for four, and oddly enough, my skills still remain the same. I keep playing in bands and jam regularly (at least 1-3 times a week for about 3-4 hours). For some reason however, I can't seem to get back into rhythm, scale, and other technique exercises for at least 2 hours daily. Is this just a phase, or should I just force myself back into the habit?
also: I recently got an all-leather strap, and after about an hour and a half of playing while standing, my shoulder gets sore, my bass isn't heavy at all (just a fender jazz) so why is it that it keeps getting sore

? It particularly happens more often when the strap is shortest (which is always, unless I'm standing in for a local punk band).
Any advice would be much appreciated.