I'd suggest talking with a professional. There are a lot of potential reasons and everyone can guess, but that's not what you need. Btw, this is a common situation, especially for younger inexperienced guys... I saw it a lot in a University counseling center. Best of luck!

Honestly, I was never able to maintain good erection in a condom. Does not matter what woman I was with. That's why I mostly avoided random sex and had sex with long-term partners without a condom.

The problem may not be "her" but it is almost certainly something 'ABOUT' her or your situation.....this is a psychological difficulty wherein your subconcious is unhappy about something to do with your relationship,...or possibly the use of condoms,..or penetrative sex.....

....theres nothing wrong with your erection ability or you would have difficulty getting hard in the first place.

Try to identify what the 'evil' influence is,...maybe religous upbringing that frowns on sex.....perhaps you are deeply uncertain about your relationship developing past just sex,...perhaps you had a bad experience with condoms in a previous relationship....(??)