Trouble orgasming during sex

Hi there, first of all, here's the situation:

I'm male, 25yo, and my (now and first) girlfriend of 2 month is 26yo (female). We have sex every weekend, because we cannot see each other during the week (so we do it whenever we can arrange a sleepover). She's all fine: she's enjoying sex, having orgasms and tries new stuff when it comes to mind.
The problem is me: I haven't orgasmed a single time yet (with one exception). It's not like I cannot reach orgasm at all, because when masturbating I come every time. In fact, one time, I started masturbating while she was in bed with me and I came, but she couldn't do anything to help me and that's frustrating for her and (by proxy) frustrating to me, because I want us both to have the best sexual experience we're capable of.

Naturally, I started looking for the differences between oral sex, penetrative sex and masturbation and noticed:
- there's hardly any stimulation inside a vagina/mouth (compared to the death-grip): no pressure, not very tight (not even when she's orgasming, although that's noticeable)
- angles are something to consider at all times (when masturbating, the hand is adapted to the right angles via lots and lots of training, while during sex the wrong angle prevents from deep penetration or even makes penetration painful)
- penetration is exhausting (like, really. As a man, I made the experience to always have to do all the work, as it's more difficult for the partner to get the position/angle right. Compared to masturbating, where you just sit or lie and don't have to move because all pleasure is delivered manually, it's quite different if you feel your tendoms and muscles stretch. I even tried imitating the movements when masturbating and it disabled my erection. MAybe because of the blood-flow?)

What I've / we've tried so far:
1) no pornography, no masturbation, only weekly sex as sexual stimulation
Idea: Decrease influences of masturbation-technique, increase sensitivity
Result: Decreased libido, couldn't maintain erection on top of not being able to orgasm
2) mimic moves of penetrative sex when masturbating
Idea: Get used to muscles being used during sex
Result: Either orgasm can't be reached or only with very light movements (with the normal manual death-grip for masturbation)
3) sex without condom (but birth control; we're both free of stds)
Idea: More friction, more to feel, so hopefully more stimulation
Result: Noticeable increase in quality of sex, yet not enough to make me orgasm

tl;dr: I (m25) have sex (with my gf, f26), but (it seems I) cannot have an orgasm during any sexual activity that involves a second person.
Do you have any first- or second-hand experience with that? What can I do to reach orgasm? Can you recommend other masturbation techniques (which are compatible with penetrative stimulation) I can try?