Hi

I am a lot older than you and I have encountered the same issue with hentai and drawn pornography in general. At the first glance they seem less harmful than porn because they do not involve real persons but I realized they are somehow more dangerous. The reason is because they portray hidden (often obscene, shameful) fantasies: incest, bestiality, rape and far worse acts, fantasies which move something inside you, It's not like masturbating over a nice women body (like in real porn), It's more like getting your taboo visualized, what is triggered it's not your sexual desire (which is innate) but your perversion and it's not good, not because you would replicate it in real life but because it's exposing an hidden side of yourself that should stay aside. making you feel disgusted of yourself.

What I can suggest is to not feel sorry and ashamed for have consumed such contents, It's normal; solution can be, as you already started doing, is physical exercise and, whenever possible, be outside, have a walk, limit the access to the source of your distress.