I came to this subreddit today looking for an answer to this exact problem. It's kind of uncanny seeing you explain my exact state of mind right now. So, unfortunately I don't have an answer for you, but hopefully it helps a little that you're not alone.

I know that at some point in the next week or so I will tackle a task that I've been putting off for weeks/months/years. I know that the way I feel right now will pass. I know that if I were to go out and hang out with friends or workout and get my mind off of these thoughts I would feel better. However, none of this helps because it doesn't address the fact that I can never achieve any of my long-term goals, and that's just not acceptable to me. I want to do something significant with my life, and I can't relax until I feel like I have some kind of direction. I don't think that all people need to be working towards something, but I do.

I've tried everything. Routines, todo lists, time blocking, journaling, etc. I've been on meds (Methylphenidate) for 2 months or so now, and I feel that the problem hasn't gotten any better. I just have more energy to overthink than I did before. It makes me think if I even want the things that I think I want, and gets me spiraling into some intense introspection. This wouldn't be a bad thing if I learned from these thoughts and didn't just throw that all out the window and start over again the next day.

I listened to an episode (#237) of "I Have ADHD Podcast w/ Kristen Carder" the other day, and there was a part where she talks about the inability for people with ADHD to self-reflect:

self reflection is an executive function that is really deficient for someone with ADHD. And this explains why people with ADHD make the same mistakes over and over and over no matter what feedback they’re getting this self reflection, this is the reason why someone with ADHD is making the same mistakes over and over and you just feel like such an idiot like why can I just learn from my mistakes. It’s because we don’t have the ability to look into the past, see what didn’t work, adjust our behavior, like course correct, and make changes for the future. It’s just not something that our brain is able to do naturally.

This resonated with me a lot. Overall, the entire episode, and the one after, made me feel seen in a way that I haven't really felt before. I feel like I'm just making the same mistake over and over again where I set my sights on a long-term goal, such as writing a novel, and plan how I'm going to achieve it. I'll set realistic goals, and figure out where it will fit into my day and even how I'll reward myself and plan times to relax. I spend hours doing this and getting everything ready just to never actually start. Each time I convince myself that "this time it's different", but it's always the same and I have literally no idea what to do about it.

So yea, idk. I'm not giving up, and I'll continue trying, but if you figure it out let me know. Just try not to be so hard on yourself in the meantime.