I am guessing you would be surprised to know that the "average" amount of time a guy can last continually thrusting is between 2 and 5 minutes. You say sometimes it is as low as 2 minutes, so I assume at most other times its higher. I don't think you have premature ejaculation as such, I think you just suffer with what almost every man feels, that they cum before their partner, or too soon for their liking. You sound completely normal for a male. Of course, it is possible to train yourself to last half an hour or whatever you target time is.
I think porn depicts men lasting 20/30 minutes but bare in mind, they are not constant thrusting, they are also probably not that turned on, so much so they need to stop and wank to get hard again, they have cameras and lights in their face etc, plus, yes they have trained themselves. Porn is not like life though. I have been with a few guys who have got this training down to a fine art. They could go for hours. It chafed me raw and I was thoroughly fecking bored with it. One guy would do 5 minutes in one position, flip me over, 5 minutes in another, and so on. Was I thrilled at having a partner who could do that? No I was bored and sore. He thought he was a stud. lol
Most of my partners have fallen somewhere within the 2 to 5 minute mark, that is, they would ejaculate within that time, sometimes sooner, if they did not stop thrusting. I MUCH prefer these lovers. I actually have a bit of a fetish for "premature" I find it a massive turn on. I understand in the guys head he feels shitty, but to be I feel like it is a compliment to my skills. Maybe we are both wrong in our thoughts (In other words maybe he should not feel so shit because it is quite normal and I shouldn't feel like it is a huge compliment as it would probably happen with any girl lol) but still, I find it a huge turn on to know my guy is close to orgasm while we play. It pushes me closer to orgasm too, knowing how horny he is.
If you feel like you are unsatisfactory, well, I have experienced two types of men. The ones who continue to pleasure the woman even if they cum early, and the ones who roll away and fall asleep. If you are one of the first, then you seriously have NOTHING to worry about! However, if you leave her high and dry a lot, or cannot bring yourself to pleasure her after, you might want to ensure she cums first, with oral, toys, fingers, even get her really, really close and then start penetration.
There are ways to train yourself to last longer though, if you wish, I suggest trying these techniques:
1) First, try to find a way to relax and reduce the worry about it. Its well known that anxiety/stressing about it actually can make it worse, or keeps it going. If it happens, try to brush it off and try again another time and really try not to let it bother you if you can. If you focus on it....well, it kinda stays
2) Wear a condom to reduce the sensitivity for you, without using numbing agents to numb her at the same time. I have also heard cock rings can help somewhat for some guys.
3) Learn to edge. While alone, or with your partner, practise taking yourself close to orgasm, stopping stimulation until the sensation dies down, and repeating this. It will help you find your point of no return, and help you feel more in control as you begin to be able to know when it is close and when to stop to avoid going over. Also referred to as the stop and start technique when done during sex.
4) Tone your pelvic floor muscles. It will help you gain more control when orgasm is close.
5) Squeeze under the head of your penis, or at the base of your penis, or gently pull on the balls: All of these are to be done when you are close to orgasm and feel you may be too close. Not all at the same time, but just find which one works best for you.
Then of course there is stuff like, not having sex in the position that gets you off the fastest. If you have a favourite position that pushes you close really fast, avoid that position until you are ready to orgasm. Try less stimulating positions, go slower during the sex, think about anything but sex (Something totally not sexual) to take the edge off the pleasure, etc etc.