Recently got girlfriend and now can’t get erect

I’ve been with this girl for a week now, she’s great really hot and nice but idk why but my dick just can’t get as hard as it used to and I struggle to get it hard “on command”. We tried to have sex today and I couldn’t get it hard enough to penetrate, she thinks it’s her fault so now I feel really bad, I don’t want to jump straight to medication any fixes?