I'd like to thank everyone for their help and useful suggestions.
javacisnotrecognized:
This is pretty much how I understand the concept of tension, although understanding it doesn't necesserilly mean I know how to overcome it

Anyway, you made a good point - I started to note that whenever I attempt to play forte, I keep pressing the key down until the finger is raised again. I am now making concious effort to stop this, and I think it's already getting better.
go12_3:
I like the way you explained how to use gravity to produce loud sounds, but how do you apply this technique to eg. fast scales? So far as I'm concerned, there isn't enough time to raise the hand, thus gravity is of no use. Or am I just not getting this right? By the way, I reallly like the "shake your hands" tip - it helps a lot.
8426:
What you wrote makes sense and I keep trying this, although it's easier said than done - I'll keep trying though, hopefully it won't take me too long.
qusimodo:
I wouldn't say my teacher's an idiot, I mean, he's a really nice guy, but I just have this feeling that he's not a very good teacher. Like, some time ago, he discouraged me from memorizing pieces and told me that I really should focus on sight reading, because memory can always let me down (and since it can, it sooner or later will). And once the memory lets me down, I am not going to be able to do anything (as opposed to having the score in front of me). He concluded that the only benefit from memorizing is that I excercise my memory. I like memorizing, because I can rehearse a piece in my head even before I can play it.
Anyway, to sum this up - I think all of you suggest that I should first develop technique, and then play loud, instead of playing loud and trying to somehow miraculously beat the tension. I guess I just have to keep working, that's all.
Once again, thank you for your useful tips.