I had a similar problem - was able to retract foreskin only when flaccid. After lots of research, decided to try stretching and after a lot, and I mean a LOT of trials (manual stretch, stretch when erect, foreskin balloons yada yada etc etc) the method that worked for me was to use a set of progressively larger silicone rings to gently stretch the foreskin... Similar to what people use to enlargen their ear piercing. The one I bought can be found
here. Its a nice small kit and very convenient.
It took about 4 months but I can now happily retract when fully erect... So plenty of patience required
Disclaimer: This isn't an advert, just sharing my experience. Interested bro's can pm me for more info.