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I have the idea I can control them when laying or sitting down, but not when I'm performing the sexual act with my entire body...
Yup. If you notice, when you do almost anything that increases the tension in your body, your PC muscle is one of the first that will involuntarily contact to shift the tension somewhere else. Doing positions where you're as relaxed as possible first will help you get into a nice groove first.
By doing kegel exercises, you can feel an increase in your ability to keep your PC muscle relaxed.
I do kegels all the time, every day, whenever I think about it. You can do them sitting down at a desk, whilst walking or waiting in a queue, when taking a piss (stopping and starting abruptly). The longer you can hold each one, the stronger it becomes.
The other part is a simple visualisation exercise. When you're "practicing" feel yourself get completely hard and then hit a pace where you're aroused, but in no danger of blowing anytime soon. Call that level 7 (where 0 is completely soft and you're sitting down doing a tax return, and 10 is no matter what happens, even if a train were to hit you right now, you're coming). The idea is to feel what it's like to be around that 7 mark and adjust your speed and depth of stroke to stick around that level. As soon as you feel yourself creeping towards that 8, just back off slightly on the pace, or stop completely, and then continue once you feel you've relaxed your PC muscle and are down to a 5 or 6. After a while of getting to 7/8, then backing off, then continuing, you will de-sensitise to that speed and will be able to slowly increase the pace until you can pound as hard as you like for 20-30 minutes straight.
The final part is to make sure you give her at least one orgasm before you even start penetration. Lick her pussy, and then after a while you can insert a finger about 2 inches in and do a "come hither" motion pressing on the upper wall of the vagina, where the texture is different, gentle at first and then with more pressure and slightly faster. Together with licking or fingering the clit, this can give her both a clitoral and g-spot orgasm, either one after the other or simultaneously. Inserting your finger about 3-4 inches in (almost as far as it'll go) WITHOUT hitting the cervix at the back, and doing the same come hither, will give a deep spot, full-body orgasm. After that, you can penetrate and she'll be warmed up and more easily have an orgasm, but even if she doesn't you take all the pressure off of yourself to "perform" because she's already had at least one and is happy!
Believe me when I say that, if you can last when you masturbate or have a blowjob, you can also last as long as you want during penetration if you just do the simple exercises.