it may not be working for a number of different reasons (and TBH, I never do this clinically, because I have never gotten any lasting results from it):
first, it's very hard to maintain a fixed point on one end of the hams for the stretch as both the knee and the pelvis are moving; when you flex the hip up past the available range of the hamstrings, the pelvis tilts posteriorly (flattening the low back into the floor), and then lumbar spine flexes; at the same time the knee usually starts bending; this typically gets no results, because you have no fixed point of stability for the muscle to stretch off of; also, while you may get some temporary increase in flexibility, if the hams are "tight" because they are compensating for some sort of pelvic core muscle imbalance when in gravity, they will often return to that functional length if the original imbalance still persists (just an example, not saying this is the case with you)
also, one should really not ever feel "incredible pain" when stretching, as this will most likely make the muscle contract reflexively and then you end up fighting yourself (an exception might be when stretching out contractures, but this is clinical, not recreational)
it may be a question of expectations as well - if you have always been tight, then your progress may necessarily have to be incremental; and also what one does in between stretching can have an influence on retention of gains made in training
if you are looking for someone to work it with, try to get someone who is trained - either a certified personal trainer, athletic trainer, etc. who knows about contract / relax techniques - this can at least help with the neuromuscular part