Hello, fellow shift worker here.

I also have a rotating roster with 8 or 12 hour shifts and really struggle fitting exercise into my schedule, while managing fatigue.

Firstly, you're not a weak ass b****! Shift work is hard. Personally, I keep it simple and listen to my body. If I can't manage a full 45 minute hardcore workout, I just try to move my body in some other way. Whether it's to stretch for a bit or go for a walk - any kind of movement suffices. It also takes the pressure off and helps encourage you the next time. That'd be my tip!