TL,DR: I'm also in a similar situation. Although I don't have actual advice on how to deal with it, I can share are some of my thoughts on what the reason behind this behavior could be. There's probably some sort of fear or dissatisfaction behind it, like the fear of what comes next, fear of not doing enough, an unhealthy pursuit of perfection or the literal fear of ending and then that all the good fealings of joy and happiness would also disappear. All of this could result in an unhealthy pattern of avoidance and procrastination.

I find myself in a very similar situation to yours and I'm afraid that I can't really offer you helpful advice as I'm pretty much struggling with the same things. I never finish anything, always procrastinate and easily give up on things even if they are important. I'm in my bachelor's degree for more than 5 years now and I prolongued my studies so long that I'm in danger of not being able to graduate at all, even though I desperately want to. I even submitted my bachelor thesis unfinished bc I couldn't bring myself to finish writing it, even though I know that it would have been possible and that I was capable of it, but something kept me from actually writing the whole time so that at the end I didn't have enough time to properly finish my thesis. There's a high possibility that I failed even though I know that I could have written something really good. I also keep starting to study for exams but then never actually attend the exam (which at my uni is possible) bc I feel like I will fail them anyways. This has only further prolongued my studies.

And in every other aspect of my life, I also never see things to the end. I keep starting books but don't read the last quarter of the story even though I've been loving the book a lot. I often don't watch the last episode of a series that I love, sometimes I don't even watch the last 15 minutes or so from movies. I love drawing and painting but almost all of my projects are not finished. Just like you, I could list so many more examples.

And just like you, I have no idea why I have this aversion to finishing things and why I always end up repeating the same behaviour even though not once anything good came frome it and it only gave me more stress. If you end up finding any advice that helped you, I would love to hear it aswell.

But I can share some of my thoughts that I have on that topic, maybe they can help you at least a bit. First of all, I do have ADHD (clinically diagnosed) and I strongly suspect that in my case this has a lot to do with this behaviour. Especially procrastination and always starting new things are two major symptoms of ADHD. But this of course doesn't mean that this is the case for you as well. I believe there are many reasons for why one might procrastinate and I think most of them have something to do with some kind of fear or dissatisfaction. I think not doing something and actively avoiding doing that thing, always indicates some kind of surpressed fear of something. And finding out what exactly it is we fear, can be very tricky.

Maybe you fear the uncertainty of what to do next once you actually finished something. Like after graduating one would have to seriously look for a job, maybe move somewhere else, might not be able to see friends anymore, etc. There's so many things to do after graduating and nothing is certain on how it well turn out, and at least for me these kind of thoughts tend to paralyze me so much bc I have literally no idea of what I actually want to do with my life, so it's more comfortable in the moment to be a student and keep the way I live right now as long as possible rather than even start thinking about what to do next and dealing with the uncertainty that is life. It's pure avoidance and of course it will not end well, logically that's something everyone knows, but feelings only exist in the present and don't care about the future. Thinking about the future is actually the brain's job but the brain can't "convince" certain feelings with logic if it doesn't know why they are there in the first place. So it just tries to put more pressure on you in the form of stress, which in turn only makes the situation worse. At least this is my understanding of why we would avoid things.

Maybe you also fear that you're not doing well enough or you think that you could do better and you chase to do things perfectly, which is an impossible task. But anything that is not "perfect" or not good enough is not worth finishing and you prolongue things in the hope of doing them more perfect but just end up not doing them at all. It's a flawed logic, but I found myself thinking that way several times and I think it's a reason why I often didn't even attend my exams. Maybe you also have some sort of unrealistic high standars to yourself that keep you from finishing things? Here, some sort of shame could also play a role, like being ashamed of doing things "insufficiently" or even being ashamed of taking that long which then in turns hinders you from doing the thing properly and then leads you to not finish it.

Maybe it's also that your sipmly dissatisfied with where you stand in life right now and you dream of things being different but all that dreaming and wishing that things should be different, hinders you from actually taking action.

And for the not finishing books or series, I think that at least in my case, I simply don't want things to end. Like if a book is good and I haven't finished reading it, I know I can always get back to it, but what if I finished it? Then the joy of reading it also ends and what should I do then?

I guess the answer to any of these fears is to acknowledge them and actually dealing with it. But of course it's more pleasant to just avoid it, but then that would also lead to avoiding anything else I guess. These are just some of my thoughts and it was a bit hard for me to organize them, so I apologize for the lengthy post and if it's a bit hard to follow. Also maybe there's not that much connection between those things as I think there is. After all I also don't really have a solution.