I know how strong I am, because most of my weightlifting takes roughly zero skill: I squat and I do standing shoulder presses. The other lifting I do is clean and jerk, which takes strength, but also technique, timing and balance. Not to mention, a bit of nerve.
The last few months, I've been struggling to C&J 145 and 150 but I know that I'm just as strong as I was when I routinely did 155-160. So what's the problem?
Some mechanics: The bar starts in front of you, so your knees are hanging over the bar. By the middle of the clean, the bar needs to be touching your thigh and your knees are on the other side of the bar from the rest of your body. I've been focusing on form--once the bar clears your knees, you need to pivot your hips toward the bar as you unbend at the waist, so that your shoulders move to vertical. The whole idea is that your body's center of gravity needs to be identical with that of the bar, otherwise you can't really drop under it to catch it on your shoulders (front).
The problem is that I was neglecting my timing: You shouldn't be going "all-out" all the time. If you pull too hard at the start of the lift, your legs will overpower your back and round it. Then you'll have no power later--and might injure yourself.
I did a timing re-set last weekend: I normally would do; 115, 125, 135, 145, 150, but this time I did; 115, 115, 120, 120, 125, 125, 130, 130, 135, 135, 140, 140, 140, 145, 145, 145. I never went very high and graduated the weights slowly, so that I could get used to; lift-off smoothly and with my shoulders low, then explode with the bar at about knee height. This weekend I did; 115, 125, 130, 135, 140, 145, 145, 150, 150, 155.
In a perfect world, I would get right to 155-160 and do a bunch of lifts there--but in this real world, I need to have patience and get there when I get there. Don't move to the next level until the one you're at feels solid.