@anamur-- First of all, stop worrying. This is precisely why you are having performance anxiety, because you are too worried about getting things right. You are putting pressure on yourself and stressing yourself out.
You need to just stop thinking about it. Don't think about what your partner is thinking and what will happen if you don't perform as expected. Otherwise, it's going to become a vicious cycle.
If you just concentrate on your partner and the feelings you have for him or her, it will be a lot easier. And you're not the only one going through this, performance anxiety in men is very common.