I'd start by taking your time. When you start to feel yourself getting too emotional, pause and come back later when your feelings have mellowed out for a while.
Or you can try spoilers. Picking a random chapter to start at or starting towards the end to spoil some of what happens helps to give yourself time to be emotionally prepared for what's to come. For example, say a manga is 110 chapters; start reading to give yourself a basis of who the characters are and what is going on, then skip to say chapter 83 and see what's going on for a few chapters (just skim through to find spoilers like "so and so dies on chapter 89"). Now you have that knowledge and can give yourself time to process it and then start the book from the beginning. This way, you won't get too invested in a certain character etc. and can read the book all the way through properly without nearly as much emotional distress. It'll still be enjoyable too, since you'll be able to put more emphasis on HOW the story and the characters get to that point.
I often get very emotionally invested as well and its absolutely nothing to be ashamed of, it can just be frustrating at times. Spoilers is what works best for me.
Idk if it's helpful since I can easily dissociate from stories, what happens in my head is that there is a switch that gets flicked when there's a scene that's too much and I see it as a image flashing by than a thing that can actually happen to someone.
Maybe you can tell yourself that the scenario ain't happening? That it's a hallucination perhaps and it's all a bad dream and things do get better?
Maybe a tip would also be to take some time to process those emotions so there's space to continue the story again. And remember that it is a story and it isn't happening to you right now and that you are safe and loved. I know it sucks to think it may happen IRL too. :,( And in the story it hurts the character(s) too.
Tbh for me, I don’t start smt unless I will finish it in one-two nights. By then if I don’t finish it, then I usually save it for when I wanna come back (usually then I reread the whole stary if that happens) or I don’t come back ever. This happens with me for “sematic Error”- I really liked it but I was slow on reading it and then just stopped. I knew it was good, and I really liked it but I just stopped after awhile. I’ll probs at some point go back and reread the whole thing some day. (╯°Д °)╯╧╧
How do I disassociate myself from something that I'm reading? I feel too much sometimes it's tiring to finish a series. I have tons of mangas manhwas and novel that are left halfway cause I'm too pussy to continue when its getting to the climax or anything big revealed cause I'm very tired emotionally lol