Need help

I'm 34 and been dealing with pcos since I was a teen. I am at my heaviest 285lbs. I've been on a low carb diet for about 3 weeks. It's been rough and I can't get motivated to exercise. Ive been sticking to a 1500 calorie diet with only having protein and veggies. I think I'm doing OK on the food part but I have a bad knee and siatica in my lower back and when I work out I end up being in a lot of pain. I just feel like I'm stuck. I'm trying to change but it seems like I'm not getting anywhere.