I've noticed people often assume they are eating more than they actually are when they can't seem to gain.. Cutting out sugars, grains, pastas, etc will eliminate that constant need to be fed & you will be satiated more now.. That often works against people like me & you bc we need the calories.. I'm similar to you, fat burner, calorie burner, hard gainer, thin, All those adjectives! BUT, you NEED calories (assuming there is no underlying thyroid or dysfunctional gland activity) if you're stuffing 3k calories a day in your body (clean primal food) you will gain weight
When I first began my primal journey, my body dumped tons of water, inflammation, unraveled digestion congestion & leaky gut-ness from YEARS of damage & torture created by my SAD (& grain heavy bodybuilding diet).. After these eliminations took place, my body calmed, healed & began repair & rebuild.. I too was shocked @ how thin I got & panicked, but it all makes sense now..
I still think we need calories.. I sneak them in with some dairy bc frankly living off ribeye & New York strip gets expensive.. So I elect for some dairy to boost calories.. Avocados, eggs, almonds, heavy oils on salads, yogurt, cheeses, meats are all staples.. You just gotta eat more of that stuff..
I would suggest taking it easy on the cardio.. Burning calories will only chew up your caloric energy, you're looking for calorie surplus..
Nap more if you can.. I also thought the fasting improved digestion & "heal to gain" for me as well (as mentioned above)..
General theme is: more calories, less burning
Also, if you're battling diahrea type stuff, watch the "add more carbs" suggestions.. IBS-D may be more your "type" condition, & more carbs like potato, fruit &/or general primal starches may run through you, as they do me.. My body prefers boosting the fat numbers for gains
Good luck!
oh & stress will make it worse! (this is where the naps help)

ha, I know it's tough, but "Don't Worry, Eat Happy" maybe is your new theme
