Controllable premature ejaculation

[plentyofwater]

Hi, I've always had a premature ejaculation problem, which actually came (no pun intended) as kind of a surprise during my first sexual encounters in which I found myself unexpectedly getting over excited and coming too fast.  Since I got a late start in my sex life, I've always chalked it up to inexperience.

This was 7 years ago and although I haven't overcome the problem, I've found my way around it. Even though now I have a lot more experience, and I still can't help coming too fast if I get very excited.

Now I have my first steady sexual partner, and throughout the many times we've had sex, and also during masturbation, I kinda figured out how not to come too soon.  So what I do is I start normally, and when I'm on the very edge of coming, I stop (or tell my girl to stop) all stimulation, and I kinda stop my own ejaculation by clenching my PC muscles as hard as I can.  I hold it like that for something like 15-20 seconds until it's subsided, and that's it! After that, I can keep on having sex for as long as I want.  Usually I can keep going for 20-30 minutes until I'm about to climax again, and I just repeat the procedure and keep on going.  I've had sex for over an hour like this, and it's great.  And actually, a couple of times it's even been difficult to climax even after a long time, but eventually it happens.

Now, I've never talked to anybody about this, so my question from all of this is...is this in any way normal?  Could it maybe be harmful in some way? I really don't understand why it happens that way, I just know that it works.

[plentyofwater]

Anyone?