If you have both fingers - or thumbs - fully
inside the foreskin so that they are beside
the glans, than you have them positioned correctly. You do need them to be "hooked" just inside the tight part so that they will not slip out and defeat the purpose of the stretch, so they will
not seem to be applying as much pressure on the narrow area as the part alongside the glans which has been stretched out by naturally frequent erections. That's OK as long as you do
feel tension on the narrow area.
(You meant to say 10 to 15 millimetres
down from the tip!
If you use fingers rather then thumbs to stretch, you can use more
than two fingers - with as many fingers as possible packed inside the foreskin, you do not have to use your arm (or finger) muscles to hold the stretch, so it becomes much easier. And doing this will make the bundle more evenly cylindrical, focusing the stretch on the narrowest area.
We pretty much always
presume that if you could never retract before, the frænulum has not been required
to stretch, and it will
therefore need to be stretched.
If you traumatised it by forcible retraction when it was not yet retractile, it's your guess as to which part you might have injured. It is
the case that sexual excitement (including masturbation) tends to suppress pain sensation and you may only discover minor damage afterwards!
And retracting a tight foreskin is in itself a common form of masturbation.