Late_Bloomer,
Just to add a little to what's already been said, all of which seems good to me. I wonder if you might do better to concentrate on alleviating the pain for her during sex, more lube, more foreplay etc. ... but maybe if she is a relatively new partner for you, some natural widening out and gentle stretching will ease the problem over time.
I wouldn't mind betting that Donk and littlbigguy are right, and that anxiety about the pain you are causing is the root cause, and the delayed ejaculation (and consequent increase of the pain) is the result.
Like two_fister, I have a range of experience on this one. From time to time, my problem has been coming far too quickly, even on occasion the act of insertion itself will bring me off ... but to be fair this has been balanced by my ability on almost all such occasions to stay hard, and the need to come again with the same erection. Yet another reason never to use condoms!
But as other lpsg guys with this need will know, I imagine, the second ejaculation often takes a very LONG time to happen, and there is likely to be soreness on both sides by the end. I tend to swell up beyond my standard erection size in these circumstances, and while that makes me feel good, it makes the length of time it can take that much more of a test for her. Sometimes, not often I am glad to say, we have just had to admit defeat, I will pull out and deal with it myself. I hate having to do that when it happens, but I wonder if you were willing to do it on occasion, it might defuse the whole situation for you.
My other tangle with delayed ejaculation was way back at the start, and entirely psychological. It was bad enough to be almost total ejaculatory incompetence ... something in me couldn't "let go" enough to come inside a woman, all bound up not so much with my size as with the force and volume of my ejaculations. I suppose I felt that it was unspeakably gross to release that amount inside someone. Fairly quickly, with a lot of loving help, I got over it!
Your problem might be like either of the ones I have described or different again. What about taking the anxiety out of the situation by withdrawing for a bit, maybe after she is satisfied, then relubing, reinserting a bit later? Or just being very still deep inside her, for a long time?
A huge erection that won't go down can be a problem ... but believe you me ... there is no shortage of great ways to exploit it