NSFW erection/uncut issue

Hey I'm a 23 year old (m). I have a boyfriend (22) and in our sexual relationship, he tends to top more, but he is open to bottoming for me. We've been together since October and the few times I've tried to top him, I've been unable to maintain an erection.I'm uncut and I have a very sensitive head. Between the sensitivity(sometimes discomfort/pain) and sheer pressure from penetration-or trying to penetration (after helping my partner loosen up), I lose my erection.I've started leaving my glands exposed during the day and that's helped a little in the pain department, but I'm struggling to figure out this issue. My partner's been great and supportive but I'm just frustrated.Any advice or suggestions would be very helpful. I've been looking into cock rings as well. Thank you!