Not sure you're being clear. You seem to indicate you are getting erections fine but sometimes they are harder than other times. That's normal. Erections depend on lots of factors, such as how turned on you are, how tired you are, how many times you've had sex in a short period of time, age, etc. These are all normal. When it's a problem is when you can't get one. Best advice is, if you're able to get an erection, don't worry about how hard it is. When things are working, don't try to fix what ain't broke. The more you dwell on it, the more of a problem you will make it. But if you are having a problem getting or keeping an erection, there can be physiological reasons for it having to do with blood flow, nitric acid levels, prostate problems, etc. If you're young, which is what it sounds like, it's highly unlikely you'd have any of these problems and it would most often be psychological. Sex isn't a contest, it's something we do to have offspring and because it feels good.