Well now, there's
an interesting set of observations!
A couple of things come to mind. If you say you have actually
had your foreskin retract during intercourse, then it clearly will
stretch considerably more than the "5 mm" that you indicate. Also, you presumably know how to pull it forward again even when stretched to this degree, so you are safe from "paraphimosis" (as long as you remain sober!).
I think you need to pull the foreskin forward as Jim describes, but with one
hand, and whilst doing so get a finger of the other
hand down through the opening. Lubricant will probably help (as long as you have a good grip first on the end of the foreskin you are holding on the stretch).
Having demonstrated this to yourself, the "exercise" becomes to get the largest
finger (which is of course, eventually your thumb) inside and do this as often as possible and for a reasonable length of time (ten minutes or so) on each occasion. Once you can do that easily (the concept being to make it uncomfortable, but not painful), you can then
move on two (little) fingers, and move on from there.
Now as regards the condom; unless you are married I would think there is an absolute need to "use condoms all the time during intercourse" for other
reasons in any case, without fail! Of course, what I perceive you mean, is that the condom takes the brunt of the force tending to retract your foreskin during intercourse.
No doubt your partner, also 18 years or so old and never having birthed a child, has quite tight vaginal muscles bit even so, I suggest the issue of "lube" is critical whether or not
you were to use the condom. The fact that there is enough "drag" to pull back such a tight foreskin, suggests to me that even with a very tight vagina, you are having a lubrication problem which in turn suggests that she is not quite
"all right" with the whole business and thereby not lubricating herself effectively.
Of course, that is a self-perpetuating problem - if you are experiencing discomfort (from forced retraction, or sufficient to force retraction) you may presume she
is also experiencing discomfort, though she may not mention it as such, and if experiencing discomfort, being anxious about the situation, and if anxious about the situation, not lubricating properly and so the situation becomes worse. If nothing else, you should be using artificial lubrication as well as
the condom, and need to be very sensitive as to her comfort (and listening for clues she may offer about this).
So you have plenty to do - do keep us informed.