I am so sorry that you experience such frustration.
I've tried chicken breast (organic, boiled) successfully, twice. It might be easier for you than tuna. According to "lists" chicken is more alkalizing than fish. You could try hiding 1/4 cup, perhaps, of plain shaved "bits" in one of your "burrito" creations. I know that you crave protein.
Perhaps focussing too much on the numbers is doing more harm than good? Truly, IT TAKES A LONG TIME (yes, YEARS) to gain weight. Honestly, it took me years to get to a healthy enough weight (and now my jeans are way too baggy again & I wake up starving at 3 or 4 a.m.) so I know I need "more" too.