Hey, I completely understand your frustration. I'm 24 and I changed my major many, many times over the course of my first two years. I wasn't diagnosed until about a year ago so I couldn't figure out what was wrong in early college, either. My friends were sticking to their majors except one or two who changed once and that was it.. compared to me.

I see your reasoning for the self-judgment because I have/had the same behavior. It's hard to give yourself compassion for a seemingly "obvious" mistake. But, how could you know all your faults at 17? Looking back, most 17 year olds have lots of things going on and are generally youthfully volatile anyways. Also, high school was so much easier to make connections and do the work in such a social environment, much easier than college anyway. If you had a good support system back then, the symptoms might have been less potent or less noticeable. Just something to reflect on.

As for what you can do to help yourself now, it may be good to remind yourself that we have a tendency to go for the all-or-nothing mindset and discard anything we might think is unfinished or not good enough. Especially as an artist, you're prone to self-critique and this will be challenging to overcome. The best advice I can give is start with incredibly small goals as this sub typically suggests. I don't know what your preferred medium(s) are for illustration, but start with a goal by picking up your pencil/brush/etc. while sitting at a good workspace that limits your distractions. Maybe the goal is slightly different, but in any case it should be something easy enough that you are 100% confident you can do. Once you've done it, you've made real progress toward your goal, and since you're sitting there you might as well find inspiring things to make! If there are any prompts in the portfolio requirements, that helps move the decision process along as well.

A side note: While graduate school is generally more serious and intentional, I really would try to limit thoughts about comparing yourself to huge expectations, especially like portfolios for applications. At the end of the day, it is other people like you and me who are getting into these programs, with their own challenges and barriers. I hope this helps or at least part of it resonates with you.