I had this exact problem. Got to 16 years old before I realised I had phimosis or that it was even a thing. Managed to stretch it out myself over a period of a few weeks (mild case I guess) and yeah the sensitivity was insane. It really didn't take too long for it to normalise though, maybe two months post being able to fully retract the skin? It loses a lot of sensitivity rubbing in your pants and shit.