Since you rose to the occasion in previous relationships, I have to assume that condoms weren't part of the picture. You aren't alone.
Lots of men report that they experience reduced sensations with condoms. You also say that you are having trouble becoming aroused from oral sex. This is also not uncommon.
You know the saying, the hand is mightier than the sword, well I ammend this saying to: the hand is mightier than the mouth. By this I mean that young men often become accustomed to manual stimulation (through masturbation) and they find it difficult to adapt to oral stimulation, which is softer.
Many young men will also find it hard to adapt to the softer sensations of the vagina (as compared with the hand), which means that they don't feel adequately stimulated in intercourse, let alone intercourse with a condom.
In your case, since your previous lovers didn't insist on condom use, you haven't had to face this problem up until now. From a mechanical point of view, you can certainly encourage her to suck harder and masturbate you more forcefully.
You can even use aids (visual, verbal, fantasy) to heighten your arousal during intercourse. You can also try other sexual positions, which will heighten your level of stimulation.
For example, the rear entry position prevents the penis from entering the vagina as deeply as other positions. Since the lower third of the vagina is smaller and tighter, this should heighten your level of stimulation during intercourse. She can also learn the Kegel exercises (see my Advice Archives). Strengthening her pubic muscles will enable her to squeeze your penis during intercourse, which will intensify your sensations.
So far I have focused on the nuts and bolts (if I may say) of sex. Realize that even if you improve her technique to the point that she becomes a human belt sander, there is much more to this story than simple friction. You also need to determine if emotional factors are contributing to your lack of sensation.
There can be many emotional conflicts that can actually cause your body to hold back. Fear of intimacy, fear of commitment, fear of rejection, and fear of abandonment could be causing your diminished sensation. You would do well to explore if any of these options apply to you.
I say this because if you don't uncover all the possible causes of your problem and resolve them, you will still find yourself at sea. In other words, even if your lover learns how to stimulate you till you get a rug burn, if emotional factors are causing you to cut-off, no amount of rubbing will override the emotional blocks that may be causing the diminished sensations. Why do I suspect emotional factors may be afoot?
I noticed you said that you tend to have quick relationships. When a person's relationships fall into the 'eat and run' variety, it's time to wonder whether there is fear of closeness, commitment, etc. . Check out this possibility.
As for your girlfriend. She needs to take her ego out of your drawers. You need to explain to her that whatever your issues are, they predated your knowing her. Encourage her to find out why she needs to insert her ego where it doesn't belong. Does she suffer from low self-esteem? Is she using your sexual slump as amunition to pound her psyche deeper into the dirt?
Talk to her and make her aware of this issue. Remind her that your sexual experiences to date (masturbation and unprotected intercourse) have not trained you for the softer stimulation of oral sex and intercourse with condoms.
Encourage her not to blame herself, if she can help it. Reassure her that you are examining your own emotional issues. In the event that you come up empty in that department, then tell her that you will need to train your body to respond to new and different sensations.
Tell her to be patient and to have fun practicing with you.
This problem can definitely be resolved, so hang in there.