Hi there ~
I understand your concerns. However I am assuming that when you mention sexual power, you are referring to poor erections that you are having rather than decreased libido or problems with ejaculation. However you also mention that you want more time for sex with your girlfriend, which makes me think that you have a problem with premature ejaculation
as well. I believe that you are having problems in areas of sexuality that an expert in the field like a qualified sexologist
or a psychiatrist
will be able to help. However, you need to relate a good history and have a physical examination performed along with testing, both laboratory and imaging to get the right treatment for your condition. I hope this helps.
Take care and have a lovely day!