That sounds like a mental issue, not a penile one. Like you need constant stimulation for pleasure. Do you watch a ton of porn on top of sex?
Also, that thing about porn stars going from position to position quickly is an editing thing. Porn irl doesn't work that way. The video editor changes things in the video to make it seem that way. Porn is supposed to be fantasy, not a representation of reality. Maybe you're subconsciously holding yourself up to be like a porn star, but you think you're not living up to it, so you go soft really easily?