Erection problems

I'm a 24 y/o male and I've been in a relationship with my girlfriend for 4 months now (I've been single for the past 3 years before). At start our sexual life was great, everything was perfect but in the past month I can't get myself to an erection. The biggest problem lately is that I get a loss of libdo quickly. Whenver we make out and when we have sex, my boner lasts like 5-10 minutes and then problems start. My penis gets soft and everything ends there. I'm not sure whats the problem, we're trying to figure out as it is getting really annoying. I believe everything is in my head as lately so many random thoughts are going through my head when having sex..that's probably the issue as her body still turns me on and I find her really attractive. I can't clear my mind and I guess I can't relax and let everything go out of my mind..and I don't know how to make myself do that. We've tried having sex in silence, during music, during a movie, same story all the time. I don't masturbate at all so I have no problems with the "death grip" or whatever people call it. We're switching positions, we're trying everything, blowjobs etc., same story all the time. Any advices on how to clear my mind? I try to focus on my feelings, I look at her body etc. but nothing helps.

Help please

Although you may think that this is a mental issue, when was your last major physical?
I really would start there and rule out any physical problems first. Blood pressure, prostrate, circulatory, etc?
If your mind is racing or cluttered, mention this at the appointment. You may be referred on to a specialist for more tests.
While certainly not a routine issue, you are not alone with this issue. It happens more than you may think. I would let the experts diagnose this.