Hi, Please Can You Help? I Have A Situation Where

Hi, please can you help? I have a situation where I get errections during the morning where I am able to have decent penetrative sex but when it comes to the evening I struggle to get an errection with my girlfriend and it’s progressively got worse to the point of not been able to get an errection at all now. I struggle with my thoughts about what I should be focusing on and what I shouldn't do in a sexual situation in order to get an errection. I seem to struggle with arousal in the evening and pretty much in the morning but somehow manage to get erections in the morning probably due morning wood, I never seem to feel horny if that's the word I’m looking for. Just wondering what you thought? I've had doctor’s checks and everything seems to be ok but something makes me think I may have low testosterone even though it’s been checked and it’s ok because I seem to have a low sex drive especially in the evening.