I actually used to have Phimosis. I just jacked without retracting. Now, I've had a Dorsal slit to save my foreskin, and it's much easier.

Easy, just do it. You don't need to retract to jack! Or simply get a blowie instead!!

Depending on if your shaft skin is still "loose" when you are erect, you can either still slide it up and down or use a lubricant--many of which are available on line or at any pharmacy over-the-counter. Something else to explore, if you wish, is enlargement of your foreskin opening. Again, the advent of the Internet brings HUGE amounts of data on doing so to your fingertips. Many have a fetish for phimosis and would love to have you keep it tight, but it's your cock and skin to do with as you wish. Also, at your age I'd think you would have cum across any number of ways to masturbate that work for you!!! All the best!!!