Gaining

Two biggies in processed food can be salt ( blood pressure impact) and sugar ( sugar highs and lows, messing with insulin levels and sensitivity, etc).

Reducing salt and sugar will probably help on their own. But some people (I'm one) don't digest large amounts of fat well, so that is another possible factor.

Mind you, I also have digestive issues with a bunch of healthy foods, so individual bodies will vary. Basically I'd say making individual changes and holding them for a couple of weeks can give you an idea of the specifics of what works for you.

I'm struggling with that thing also and I don't cook much, therefore I eat junk I guess...

It helps a little to follow sleeping shedule strictly. No under- or oversleeping (If you wake up on your own and look at clock - get out of bed). But still that doesn't feel like enough.

Anyway I already know that its all pretty much about anxiety disorder I have. My body reacts that way - I get very sleepy when I'm stressed or anxious. And that leads to a funny counter-reaction - a good strong tranquilizer(!) literally revives me. And I feel like I can run on ceiling if I take more of it.
But usually I'm not using it, because meds-only is not the right way.

So I have another argument now to review my diet towards clean one. I still keep wondering sometimes - what crap am I eating.

Eating tons of junk food is definitely fun, but it works different for everyone and can lead to lack of energy, especially when combined with a more passive lifestyle.

You can eat clean and still consume tons of calories. Maybe you just need a feeder / chef to help you out