you just might need to dial back the power, it could be the result of damage, but it you are not damaging anything it could just be general weakness, vocal muscles are still muscles, for example, you are not going to bench press 200 over night, you have to gradually build up to it, if it does not hurt try continuing for an additional 5 minutes even though it may not sound great, also do 10-15 minutes at a time 3 or 4 times a day if you can, this will get you more prepared to be consistent for longer, the sky is the limit on this too, imagine if you can get use to doing vocals for and hour or two per day, leveling up just begins to naturally happen