I totally understand where you’re coming from.
I have the same phobia, I can’t use anyone else’s toilet, can’t go at work, won’t go on holiday and can’t go when my DP of 9 years is in the house.
People tell me it’s ridiculous, everyone does it etc but that doesn’t help me, I still can’t go.
I’m getting better and you know what it took to me get better, I had a few accidents. Seriously, rather than just go to the toilet in my own house, I held it that long that it happened anyway.
So I get how bad this is, I really do.
A few tips, tap on full, or shower, window open, toilet roll down first, maybe even play music on your phone, flush as soon as you’ve been, without wiping, then wipe, flush again, hang around washing your hands until the worst of the smell has gone, spray your perfume.
Good luck.