Sorry to hear how painful your shoulders are at the moment
I've found that knowing how much exercise I should be doing, and whether I should be doing a certain exercise is something that only I can judge. I need to feel reasonably in control of the movement, and I only feel comfortable exercising when my joints are in position. If I'm not in control - chances are I'll cause myself damage that results in pain. If my joints are not in position, I'm probably not using the right muscles for the movement, and they may not be able to function properly, this means that the exercise is not doing what it is supposed to, it's not beneficial and ... more pain.
I don't know how you can do weight bearing exercise with your shoulders when they're as bad as you describe. I think I'd ask the physio for lower level exercises.
It may be that you have some over-tensed up muscles that are pulling your shoulder out of alignment, making it painful when you try to hold yourself in alignment? Like you're working against your own body? For my shoulders, I had tense areas under my arms that caused my shoulder blade to try and follow my arm when I raised it, my chest muscles were too tight so pulling my shoulders back was hard and quite uncomfortable. Maybe you could ask your physio to check this and maybe get some stretches to do to release your shoulders a bit before exercising to strengthen the weak bits?
How about... instead of pushing your body off the wall, use your arm in the same position to push a door (not a heavy one!) open and closed? Instead of using the resistance bands, just move your arm in exactly the same movements instead? Imagine that you're moving through treacle, and hopefully, you'll be able to control the movements well enough that the exercises will be beneficial without causing a flare up. My favourite physio was brilliant at scaling down exercises for me, and she'd often give me exercises to build up to, so I'd have exercises for bad days and good days, and goals to aim for.
How many repetitions do you do? One physio wanted me to do 10-20 press ups against the wall every day, it was far too much to begin with so I did 2 or 3 after breakfast, a couple more mid morning, and spread them throughout the day, I was soon able to build up to doing more in a row.
It's very frustrating to build up slowly, but it's far less frustrating than injuring yourself and putting progress back weeks or months!
Sorry, that's a bit garbled, but hopefully it's helpful, it's worked well for me any how!
edited to say even more!
Sorry if you think I'm going on!