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Definitely get overstimulated to the point of having physical symptoms. I'm glad you have a fidget necklace. Maybe you can find other fidget toys you can use as well so you have a variety of options. Another thing I do is the five senses where you list five things you can see, four things you can touch, three things you can hear, two things you can smell, and one thing you can taste (it might help to have candy or gum with you for this one). If you're at home you can also try holding an ice pack or putting it to your neck or chest to get the sensation.

I know you're not trying to starve yourself either. Some people, especially those with ASD, have "safe foods" that they have a very difficult time deviating from, so that may be what's happening to you. It may help you to have some of these safe foods in your house so you have something you're able to eat. If they require cooking it may help to learn how to prepare them. I'm not saying never try any other foods ever, though. At least try a few bites and then if you can't handle it, that's okay as long as there's something else you can eat, because fed is best. If you are able to drink some nutrition shakes like Ensure it may help you to make up some of the nutrients you may not be getting from regular food.

Big walls of texts are super overwhelming so it makes sense that you can't read big paragraphs.

I think it's worth talking to a teacher, especially since you're not fond of talking to your guidance counselors. Maybe you can kind of tell them in kind of the same way you told us.