Just keep eating tuna and you will get used to it eventually. Your gut bacteria will change and its composition will send message to your brains what food you should or should not like eating. Lack of appetite for tuna might as well be lost ability to digest it. If there is still bacteria in your gut, it will repopulate. If not, you might not get back your desire to eat tuna. Only shoving other people shit through your behind can help you then.
Though, if we are being less scientific, just get the least aggressive tasting tuna. I did not tried fresh, but I do enjoy canned tuna. Make yourself a sandwich with little tuna on it. Just for it to get an after-taste of it. Over time you will get used to it and you will be able to increase how much of it you can eat.
Personally, I cannot recall being unable or un-wanting to eat something which I could in youth. I became more tolerant of spoiled food though. I started to like marinated or fermented pickles. Though, only in a very specific way. My grandmother had an unmatched super recipe for them. She put cherry leaves, gooseberry, red and black currant leaves, dill and rheum root. My mother or the whole food industry simply are incapable of making good marinated pickles. They all taste awful to me. I also could not stand sauerkraut as a child, but now they are okey and I prefer them on certain meals just to have balance in taste.
Though, I found it hilarious that Germans are supposedly famous for their sauerkraut and sausages. In Germany I found them to be just average, but rest of their food was horrible. By the Gods, there wasn't in the history of humanity another nation which would be so incompetent at making food. Mcdonalds to me seemed great, because how I disliked German food. When I crossed border, I was so delighted to be able to buy fast food in Poland at a bus station. Cheap trash food tasted like a feast to me after being subjected to German cuisine for months.
Btw: I do not eat spoiled food from supermarket like some people. My family loves spoiled cheese either with blue fungi or one which laid on the ground being eaten by slugs. My sister loves Kombucha. This drink is for people who drank home brew kompot and thought: "It would be so much better if this drink would be spoiled". Stupid people, I will throw you some snails and blue mold from bread into it and I will let it sit open under a sun for a day or two. Here is your home brew kombucha for you. Complete with fully grown fungi like in some drinks.
As you can see, I never understood why people eat spoiled food. My brain would always go: "What you are doing? Don't eat spoiled food you dum dum". There are quite few other things in cuisine which I just do not get it. Stale bread. Indian kitchen where they put some meat into their spices. Western over-reliance on salt. Aged cheese for one too many years. Etc.