Why is it that I can retract my foreskin when flaccid and at the beginning of an erection but when I get a full erection I can only pull the skin 3/4 of the way down my glans and no further. I get 3/4 of the way down the head and then everything stops and toughens up

Is there any way to cure this and get my foreskin to retract past the bottom of the glans and down the shaft?

Everything gets bigger except the diameter of the opening. By stretching this little band, you'll get it to retract fully even with erections. Check out some of the preceding threads and try the exercises suggested and illustrated.

trying to pull your "trakkie daks" (tracksuit pants) up - or down - without untying the drawstring.

In this case, you have to stretch the "drawstring" - which you can do as described in most of the threads here, but which will take at least a little of your time and effort.