I have suspected IBS and suspected Borderline (diagnosed MDD and Anxiety) and I swear to god my stomach health is related to my mental health. Correlated, perhaps, but there's definitely some sort of relation based on my experience.
Same with the yogurt; I'm actually planning to start taking probiotic supplements to simplify that aspect, as well as a fiber source called phylum husk.
Sorry to hear about the Borderline. Just remember that mental health diagnoses are less rigid than traditional medical diagnoses. You've heard of mind over matter; mind over mind is even more effective.
Also, check out partially hydrolyzed guar gum which has some promise for IBS. It is a prebiotic fiber (feeds your beneficial bacteria) and also like psyllium husk does not produce a lot of gas which is bad for IBS.
This might sound crazy but if u are able to fast for 2-14 days straight it will help so much . It seems to have had a very long term effects .
First try doing OMAD and see how u react . Basically fast for 23 hours and eat for one hour . If u think you can handle doing that for a week. do it a few days straight
ps; probiotic yogurt makes me very happy