Update:
It's been a few months now, and my foreskin is nicely stretched. However the tightness problem has remained mostly unchanged. After doing a bit of research, I discovered it wasn't a foreskin or smegma issue at all.
Turns out, I have a tight frenulum problem. This is a bit of connective tissue joining the base of the foreskin to base of the penis, right under the glans. So whenever I have an erection, my head curves awkwardly downwards, thus tightening my foreskin at the top.
I'll try to stretch this as best I can, but I suspect that I probably will need to do a frenuloplasty at some point. Sounds intimidating, but its a relatively minor operation. If any of you guys are experiencing something similar, I suggest googling "tight frenulum" and clicking on the first link for more info. It has helped me immensely.