Hi LornaKay,
I happened upon this post and wanted to offer you some support. I've been with someone who was not "well endowed" in the past, and frankly there can be quite a bit of pleasuring even with a smaller phallus, particularly if you get very hard.
Also, sex isn't only about penetration. Most women enjoy sensual foreplay, involving hands, mouths, lots of touch and caressing and licking and sucking, all over the body. Next time you find yourself being intimate build up her arousal by pleasuring her in such ways, which may also add to your arousal and hardness of erection, then at penetration a different position. I particularly enjoy being on my side with my husband on his knees penetrating me sideways, so a different angle than missionary or rear penetration.