Bad diaper rash won't go away?

My baby is 5 weeks and a few days old and he's had small patch of a bad rash in between his butt cheeks for the last 2-3 weeks. It hasn't gone away with Desitin, A&d, airing it out, washing with water not wipes, changing immediately after a poop, vaseline, and finally I tried the method with corn starch and it seemed to be clearing up after using it for a day, but today it looks like the corn starch irritated the other areas of his butt and its super red. Please help I feel so bad for my little guy I feel like I'm such a bad mom 🙁