I don't usually let the weight upset me. I just go in and try for what I can get usually. I mean, I have a certain number of reps I want to get, but if I fall short a rep or 2 I don't stress because there are so many things going on that you just never know how your body is gonna react.
If you're having troubl with a specific weight for a single, then maybe throw 5-10lbs more on next time. Now you don't have to think about that other weight number. Or put on 30-40 pounds more than it and just do a slow negative. Well look at that! I can lower 30-40 pounds more than that weight I couldn't get! Well if I can do that then it's only a matter of time before I'll press that other weight!
See what I mean bro.
But, you may want to hit up some doubles or triples as a change. I can do singles all the time on squats without problems and make constant progress, but on bench I'l stagnate after a couple weeks on singles. But, I LOVE triples on bench and like doubles too.
Different rep ranges do different things to different people. Make sure you get plenty of sleep too.