I saw a nutritionist for a couple of times when I first was dealing with that. Biggest thing she recommended was Ensure. Lots of calories plus it has protein & nutrients. Good for supplementing food. Also helps to just eat a little bit at a time (which will up your metabolism so the Ensure is even more important) but it gets you used to eating food.
Pick high calorie foods -- nuts are really good. Eggs if your cholesterol isn't too high. Buy the full fat everything (even if cholesterol is high) -- 80% lean meat, fattier fish like salmon &, if you can stand it, chicken legs. Use sauces -- especially creamy ones: gravy, alfredo, cheese sauce, etc.
Avoid drinking water for 60 minutes before mealtime. Drinking water before meals makes you feel fuller. Don't worry about taste combinations. Whatever you like, go with that. For me, that was putting cayene pepper on just about everything. Sometimes ginger or curry. I couldn't always taste them, but for some reason the smell or color or something made them more appetizing.
Basically take a look at all the "don't"s from solid dieting sites and do the opposite. The exception to that is that you really should still consult your doctor.
Most important for me was to always, always, always remind myself that I was eating because it was the healthy thing to do. You eat to live, not because you feel like it or it tastes good or whatever. I know that doesn't make you feel like eating any more. But it is the truth. You need to take in a certain number of calories per day. Set your meal plan ahead of time and then stick to it. If you can't eat your food all at once, take a break and walk around or do a quick task for a little bit and then come back to your food.
I know it's tough, but it is do-able. You just have to start getting enough calories in you & then take your time figuring out what works for you without making you too horribly miserable.