I don't mean to sound disrespectful, but when men come to this site and seek help regarding erectile dysfunction, saying "you need to prove yourself" is NOT something they need to hear or something that will help them. In fact, you are saying something that can make them feel worse.
You are a woman, so for you, a man not being able to perform might seem like a simple task, but these men are not coming here posting about erectile dysfunction because they are scared of their first time or because they are confident with their sexual ability. Please do not think that for men it is always so easy to do what you feel "is proof" of something for their partner.
Mental? Once the clothes are off, it's down to buisness and you need to prove yourself to the girl. Not very sexy for either head.
I agree, stress can cause this as well as the other factors. Additionally, it may just be your partner. You may not be into him or her. We don't know how long you two have been together. The same can get boring for some people.
Have you started taking any new medications?
If so, what are they, or what is it?
If not, have you recently hurt that area or are you going through a lot of emotional, physical, or mental stress?