The mind has a great deal to do with how much you lift. To help that along what I do is to acclimate my muscles to the weight by doing a few warmup sets, putting on more weight and doing less reps till I reach my target weight (the one I'll do my first work set on). I'll do one rep on my target weight, then relax and get my mind pumped up for my first real work set. After a minute or so, I'll attack the weight. By doing my target weight once before going for the work set, my mind knows what to expect, and it's funny that for some reason it's way easier than the first time I did it.
Another thing I do is to keep a very careful log of what weight I did for each set and how many reps I got out of each set. I can't believe everyone doesn't do this. Most say, "Oh, I remember what weight I did last time". But surprisingly, if you keep a log, you'll look at it sometimes and say "Wow, I need to beat 8 reps on 85 lbs. I can't believe I did that before!" And that will push you to try to beat what you did last time, or at least force you to try to equal what you did last time. I usually do 2-3 sets and I see each set as an opportunity to beat what I did last time. Sometimes I beat the first set, which is great, but if I don't, I try to beat what I had for the 2nd and/or 3rd sets. If you can do this, you will empirically be gaining in strength. The proof will be there in writing right there in front of you. Give it a try, and good luck!