Suggestions on how to get more motivated?

I find that I am very lazy and I can’t seem to shake it. Even my thoughts of things that I need to do, are tired and lazy thoughts that turn in to nothing. I cant even think sometimes because Im so tired, even when I probably couldn’t sleep if I tried. I find I drift off on to other things when I try to get something done. I have a lot on my mind lately and Im sure that doesn’t help.
I am 22 years old and should not be like this. I have been to the doctor many times before in the past over this issue, to get tests done and they never found anything. Any suggestions on how to get more motivated. I know exercise would be one, but I try that and it lasts two/three weeks tops and Im back to my lazy ways.
My life isn’t a total joke, I still do the stuff that I desperately have to do, but other than that I stay home and do nothing. I feel its ruining my life because I should be doing more than I do. Also, its rare for me to go for a night on the town with friends or something. If I am spending time with friends, its at home.