What dosage did your doctor prescribe? I would just go with that to start and check back with him/her if it is too much or not enough. I am on Vyvanse and find it helpful.

RE feeling exhausted--are you getting enough sleep? I say this because it is a constant struggle for me to get enough sleep and sometimes I think my stimulant med is helping me most by keeping me awake. That isn't its purpose, though, it's supposed to help me be more focused, less distracted, etc. So I think I could help myself out a lot by somehow improving sleep. I don't think an ADHD prescription is supposed to be helping a person be less tired.