As hinted at by Kvalidir, constant loose bowels can be a symptom of bowel cancer. It's probably unlikely to be anything as serious as that but it would make sense to get yourself checked out by your doctor.
Your doctor might then want to consider other possibilities, such as lactose intolerance (which would require you to remove certain things from your diet, rather than add to it, as your post wants).
Equally, it might turn out to be IBS. (There are some dietary tips here, if that's what it is, but get a proper diagnosis first:
https://www.nhs.uk/conditions/irritable-bowel-syndrome-ibs/diet-lifestyle-and-medicines/ )