I used to avoid eating fats for a different reason (gall-bladder attacks), but this might help:
When I started RPD and switched to raw, unprocessed fat, all my symptoms vanished. For this to work, I had to be really fussy about what I considered "raw fat," meaning that a lot of processed fats like bottled oils were not safe for me to eat for some reason (perhaps heat during processing, rancidity during storage).
same here, I always had gallbladder issues with processed fats, including organic olive oil, raw hemp oil, etcetc...or concentrated non animal source fats like seeds or coconut meat.
those had to be completely eliminated for many years, I stay away from all seeds and nuts and only partake in coconut meats or avocado on occasion.
the best fats that have never caused issues are raw egg yolks, animal fats including marrow, and butter/dairy fats.
I truly think the egg yolks are healing to the gallbladder and bile production because they naturally contain many nutrients that help fat digest, such as lechitin.
I think a diet high in bitter greens like lettuce, dandelion, chicory, radicchio, endive..etc...and other soothing herbs and veggies such as fennel, dill, basil, mallow etc...blended up into juices with zucchini, cucumber and the occasional cabbage juice, garlic, turmeric, and lemon is very healing to the gallbladder.
very lightly cooked fish and meats with seaweed and curry broths...all of the spices to strengthen the digestion. trying raw egg yolks to see if they don't make you sick. sauerkraut and whey fermented meats might also be a good idea.
do you have a source for raw milk? raw whey and egg yolks were pretty much my lifeblood when my gallbladder and candida were totally out of wack. I was also extremely skinny and my body started deteriorating my teeth and bones until I started drinking more raw whey. looking back I wish I would have eaten much more raw, fermented dairy, fish and krauts and focused remineralization, sauna, meditation and sleeping to heal instead of trudging on.
also because you are underweight and have compromised digestion you are probably laking for vitamins and minerals, something that contains both zinc, iodine, chromium, among others if you are having issues with candida. dulse, kelp and other seaweeds(and oysters when you get there) are excellent for this, along with supplements, I like any of the new chapter products as they are food based and seem to have the proper forms of most vitamins and minerals along with enzymes and probiotics.
consider cod liver oil and a vitamin d supplement
a probiotic and dosing pretty high seems to help overcome intestinal issues, the best I have found is garden of life 'primal defense" which contains many strains of probiotics including those from healthy soil, which are extremely important. you can buy it in the powder form, not in capsules, and it is much less expensive and also easier to take larger amounts.
another really good healer for candida is sauna (once you get some good minerals in your body and are thoroughly hydrated) saunas can really speed the healing process by balancing minerals within the body and expelling excess through resperation. they are especially good where compromised digestion and candida have exacerbated imbalances of minerals, heavy metals and bacteria in the body.