Perhaps it's a mental thing? Here are some things I can think of from my experience:
1) Lifting by yourself isn't for everyone. I tend to be antisocial in the gym, but I could never work out by myself all the time-too much time alone with my own thoughts.
2) Analysis by paralysis, bad thoughts dictating how you perform. It's not easy and sometimes you'll have to get really mad and aggressive to make sure you get a good workout in.
3) Maybe try lifting at a different time of the day like first thing in the morning? I personally feel very lethargic and tired at night no matter what.
"First with the head, then with the heart"