The way you describe it makes it sound like a matter of choice, you just need to decide to take your games more seriously -- but I know it's not always so easy :) Even if you don't feel like it, things like stress and lack of sleep really do impact how well you're able to play... but you said it's nothing like that so...
It sounds like it would be good to try to give yourself a system of thought for each move. The point is if your calculation's are to the point/accurate, you don't need a lot of depth, and if you're forming good evaluations before you even start, you don't need to broaden your calculation either, you'll only look at what needs to be looked at. The following is basically how I try to play...
When it's your turn, don't calculate anything at first, just break the position down. Notice the pawn structure, notice which minor pieces are strongest and weakest for both sides, look at development/activity, look at space -- I'm not just making a long list, make sure you notice these things (and others like king safety etc). And also what will your opponent be playing for, what will you be playing for and on what side of the board (again actually figure this stuff out). Only then pick out a few candidate moves that promote these lines of thinking.
Then, as you're calculating, don't just look at tons of "stuff" -- I like to ask myself if I like a visualized move for my position, all by itself. eg don't calculate your opponent's response yet, don't think about if you want a tactical position or not, just look at that one move. Does it make sense with all the things you evaluated previously? Will it be a good move no matter what your opponent plays? (look 1 ply ahead at a few possible responses, including responses that ignore your move). Only now visualize your opponent's response. This gives your calculating more accuracy and you won't waste so much time calculating tons of lines that are "fun" but no good. Believe it or not, if you can go 3-4 moves deep (6-8 ply) and they be accurate/strong moves (almost) every time, you're already playing well above B class.
I catch a lot of odd moves/wrong thinking with the questions "does this move make sense all by itself (no calculation) considering what I've just evaluated" (which is basically Silman's "what wonderful thing does this move do for my position") and the question "do I like this move for my position no matter what my opponent plays"
So basically slow and accurate will do you more good than trying to take on even more calculation by going making deeper and broader calculations -- while giving yourself a system to do it may help with the discipline aspect of it.
This topic has come up so many times in so many places I thought about not posting about it here-but let's give it a shot. It is probably worthwhile because I hear so many people saying that these two problems exist for them (although they may call it something different).
In my OTB chess, I get good positions out of good openings against higher rated players most of the time. I often have a decent positional understanding of the game. My endgame technique is OK for my level (Class B USCF). AT TIMES, my tactical vision is pretty good although not good enough to take me to 2000 I think. But lately I am losing the vast majority of my games against higher rated opponents and some against lower and equal rated opponents.
This past weekend I lost 40 points USCF due to terrible play. I am sure the problems I am experiencing are due to this issue.
I am frustrated not because I dont know what my problems are, but because I know what they are and can't seem to fix the problem. My problems are as follows:
1. I have problems concentrating but I think it is a question of discipline.
2. My calculation is not broad enough or deep enough. In other words, not only do I not consider enough candidate moves, I also dont look very deeply into the position. I know I should, but I find it hard to orient my mind toward the process. I wind up playing and enjoying tactical games as it turns out so this problem is magnified.
3. I need to develop greater tactical vision.
The last of these I understand how to remedy. The first two I am failing at. I have even created a cheat sheet thinking process but just cannot stick to it. I dont think it is physical (tiredness, mental, etc). I think it is discipline and muscle reflex so to speak.
How did you train your muscle in this regard.
Looking for help (lost three out of three this weekend).
Thanks.