Worried about erection problems

We all faced it, but it's happened to me a bit more than normal I'm sure. Didn't really have that problem with my ex, but I've had it almost every singe time I end up with a girl for some occasional fun. Everything goes more than smoothly until it's sexy time. I don't know what exactly happens to me but a raging boner goes into flaccid mode in a few seconds as soon as penetration occurs. It happened when I was sober, high, drunk, tired or energetic so I'm almost sure it's nothing physical. The only way I can describe what happens in my head is some sort of self-consciousness that suddenly hits me and then everything goes wrong. I know it's far from being the best feeling in the world for the girl, but I'm sort of stuck and don't want to resort to pills or meds cause that would be treating the symptom not the problem. Did anyone experience something similar? Need advice, wisdom and help figuring out what could be causing this. Thx