Hey there!

So, I used to have this long-term girlfriend (about a year). This post is NOT ABOUT HER SPECIFICALLY - I just think the topic might belong here to save OTHER relationships. Please notify me and move this before if necessary.

It all started great, she came on strongly to me while visiting my uni, we started writing a LOT (since it was long distance) and when we first met again she immediately kissed me, cooked for me, we had sex... it was awesome.
I clearly had the upper hand in the relationship, being the better looking one, the one who had what she desired most (place in a very exclusive university, self-control, good family) and the more self-secure one.

Fact is, I was (and still am, working on it!) a "nice guy (tm)". I was very needy, always felt that I needed her to be happy, I needed her to motivate me, I needed sexuality from her in my life.
In exchange, I tried to be the perfect boyfriend, always supportive, listening, helping her with loads of important stuff, and so on.

Well, needless to say, I didn't get what I wanted, our sex life was TERRIBLE (though she admitted to being generally a person who had, due to trauma, problems with bodily closeness and I was the first one ever to make her orgasm and therefore enjoy sex - so it was not that I was bad in bed), seldom and monotone.

Trying, again, to be the good guy I tried explaining my feelings over and over to her, how much it hurt me, while being understanding and respecting towards her. Giving her "time and space to change".
Again, it didn't work, I was almost begging for some action, and got less and less in return.
It made her judge me as un-manly, while I kept explaining to her that it was her denying that made me a whimp over time.

Anyway, I ended it now.

But, I would like to know - how COULD I have changed the situation, being a sensitive, intellectual guy and not over-acting the other way?

As shallow as it sounds don't wife a girl who is uglier than you. Same for the rest. You were out of her league and even wifing her signalled that you lacked options and respect for yourself.

You haven't failed anything it's just you gaining some valuable experience. unattractive to girls unattractive to girls That's a good thing but make her appreciate it.