As I have argued before, pulling the skin back over an erection is probably not the most effective approach to stretching, so if you get slow progress, well you get slow progress. Pain is not
an essential part of the progress but I suspect that
- Maintaining a firm erection for 20 minutes whilst holding a steady pressure and not moving anything is a big "ask" and anything less than a firm erection is even less effective, and
- doing this, the discomfort that limits your stretching is not entirely from the foreskin opening but at least in part from pressure on the glans, so that you can be "wasting" the unnecessary discomfort.
Fingers on the other hand, are already "firm", and thus if you can
get at least one tightly in the opening (without
an erection, in fact), I think you will
find it more effective - and possibly do-able simply with a hand down the waist of your pants.
Beyond that, I think that any stretching is a good thing. Do remember that part of the effect here, is that you want to skin to re-grow
to the new size, and such growth does
take time (though the amount of pressure and as Jim says, the time you can devote to it, preferably spread over
the daytime, is very important. Ten minutes twice a day is probably better than twenty minutes once).
In any case, keep at it. Yes, it's not the first time we have heard this, but then those people generally come back a month or two later to tell us how things sorted themselves out in the end.
Also - you might consider what sort of smooth, plastic (non-irritating) object you might be able to leave in place for a significant part of the day to hold the opening on at least some degree of stretch. A short length of plastic tubing or piping with the edges meticulously smoothed and rounded might be possible, but I'm afraid we are not in a position to arrange anything specifically for this. (The cap from a used felt-tip pen might however be a consideration.)