Welcome. First of all, you should never ride with the choke on, always let the bike warm up until it will run without the choke. If it still won't run well without the choke, you have carb problems. Second, only use the lower gears for climbing steep hills. The Rebel has a very small engine, and the powerband is way up there. You need to keep the engine spining in the powerband, especially under load. Putting to much of a load on an engine that is running below it's powerband is called "lugging" the engine, and will destroy the bearings in a very short time.
In your case, I would also check the clutch, sounds like there is a possibility it might be slipping. If you have any flat level roads where you are, see how it performs on those.
BTW, on fairly flat roads, I usually shift it at 20,35,45,and 55 mph under most conditions, usually at wide open throttle. Jerry