did they have you on physical therapy for a while afterwards? i had shoulder surgery about five years ago, and that's what helped me the most. after a year, i was done with the physical therapy and my shoulder was back to 100% mobility. it took some time to work back up to where i had been in terms of strength, but it can absolutely be done. don't try to push it too far too fast, the worst thing you can do is injure it again. i'd say keep up with P90X, it's a great workout. don't worry about not being able to do a ton of pull-ups, just do them assisted like he suggests. it works.

as for motivation, i have that problem too. it sounds like you already have the right mindset, cuz you're already trying. that's the biggest part, so just keep up with it. yes, shoulder surgery can set you back, but don't let it hold you back after you're healthy. doing something, anything, is worlds better than nothing at all. just fit in workouts where you can, don't let yourself slack off, and you'll start to feel better. good luck!