My 9 yr old actually gets tearing down there....I use aquaphor with her. Or even A&D ointment.
That's a bummer! Okay, one weird thing to try that my doctor recommend is coconut oil. I know . . . sounds weird. But who knows. But I'd think you have to also give yourself a rest. After child birth, for example, you can't have sex for a minimum of 6 weeks until you are checked that you are healed and if you aren't, you aren't given the green light for sex. So, you may need to abstain for a bit and do 'other' things.
Wow this sounds awful. I'm sorry you're dealing with it.
This article is pretty basic, but it lists a lot of things to test for, so I'm linking it so you can make sure you've exhausted those possibilities -
https://dermnetnz.org/topics/recurrent-fissuring-of-posterior-fourchette/
One thing they mention that you don't is a biopsy. Has that been done?
I'm going to move this to Women's Health so more people will see it.