@fireball. Hi there!
I have the same problem. I do exactly as you do. I hold the foreskin back and use one hand to jelq the shaft. I usually get a bit harder doing like that. When I have a 60 - 70 % erection I change and jelq ordinary with foreskin loose. I grip as very low as I can, really grip as close to the scrotum as possible. Then I very slowly with a tight grip ( I use baby oil btw ) jelq upwards my bellybutton and stop at the glance. Then I take the other hand and with the outer side of my thumb I press the foreskin down just where the foreskin starts and the fat pad end( still holding first hand under the glance with a tight grip ). The turtle neck disappears, the penis is well stretch at this moment, and I try to again grip with the lower hand as tight to the scrotum as possible. When I have a tight grip I let go of first hand and the glance is free. New very slow Jelq upwards the belly button and so on..
Every Jelq takes about 3 seconds.
When my erection gets to low I hold back the foreskin and jelq with one hand again until I reach a better erection. Then I go over to the jelq with foreskin loose again.
I do this for roughly 20 minutes a day plus warm up/down 5 minutes each.
Having so much foreskin as we do means that doing the jelq with over hand grip is more or less impossible. I do not do the over hand jelq at all. I always grip with thumb up and pinky finger down or better explained thumb closest glance and pinky finger closest scrotum.
Another thing I noticed as I often do the jelq with an erection about 25 - 30 % is that it is easier to feel the blood getting squeezed up towards the glance if I lay the thumb a bit higher than the pointing finger when I do the grip. Thereby getting a “closer” and more force full grip. Also I tend to bend the penis outwards ( away from my body ) and getting a better pressure of the blood that way.
This is hard to explain without video but I hope you understand.
additional to jelq you should do some kind of stretching. I use an extender. That will probably also help with decreasing the turtleneck.
Ohh and one tip, not concerning turtle neck, I got the advice here from some guys to get a infrared lamp. I use it during jelq to keep instant heat on the penis while jelqing. It might help with the gaining.
/ Soulmedeep