(I write assuming you are both in a country where you can both legally consent and both feel mature enough to do so.)
Nothing wrong with you - it's normal - it's stage fright.
So you want to work on reducing pressure and increasing confidence.
1. Talk about it. It's not you or her or the relationship. It's a someone knew to the hobby building up their skill set.
2. Increase intimacy - non-sexual nudity - massages - showers/baths.
3. Accept that you are not a machine - you're a human being.
4. Practice makes perfect.
5. Mix and match switching between things that are easy to maintain and erection to and things that are 'harder' to.
6. Try to identify what's easier and what is tricker to.
7. Exercise, relax and feel good about yourself.
8. Use protection and be sensible.