The first 2-3 weeks starting up or getting back into the gym is always terrible. It sucks both physically and psychologically. I mean think about it...look around society and tell me what you think separates the 85% of out-of-shape people from the 15% in-shape people? THOSE COUPLE WEEKS!
When I used to drink a lot, I'd always kind of secretly wish that I could get the hangover done before the intoxication, just so that I could enjoy being drunk more, not worried about what's coming in a few hours. The thing about doing positive things in recovery is that you actually can get the hard part out of the way at first, and then truly enjoy it when the rewards happen.
And you're someone who's been through battles with drugs, so if you can handle that, you can handle a little bit of pain in the gym. Trust me, if you give it your all for 3 weeks (at most), science takes over and you start to feel amazing. I always struggle those first few weeks...everything is all shaky. You need to build your stabilizers up. But once you do, around the third or four week, your strength absolutely takes off. You notice yourself standing up straight, walking is easier, you have more energy, you can bend down to pick stuff up easier, and so on.