I also can relate to this.
One of the things I've started doing recently is making backtracks of songs. Someone would send me a song, and I transcribe it, note for note, instrument by instrument, but omitting the vocals or lead instrument. I know a friend who makes money producing such backtracks for clients, and so that inspired me to start doing them for live performances I do. One day I might try to make a business on the backtracks themselves and not on just performing, but I don't know if I could sustain the backtrack creation long enough. I burn out easily.
Unfortunately, the process of doing a backtrack brings out the anxiety over taking on large/complicated projects. When I start a backtrack, I am initially freaking out because there are so many instruments, so much nuance to try and get down. When I started doing these backtracks sporadically about two years ago, I started by first transcribing the easiest stuff for me to hear, which was often bass. Then when that was done, I'd go to the next obvious, which was usually guitar, and then drums, and from there the order is anyone's guess.
But I still freaked out constantly for two reasons. A, I spend so much time and effort on this process that I exhaust myself before I have much to show for my hours of hard work. And B, there's a chance that my hours of hard work will not even be worth it because the end result will sound horrible, and I won't know why. In fact I am so afraid of this now that I am afraid to start any new tracks. These fears are real possibilities and are too much for me to bare. And, if I have to do multiple backtracks in a set, a third reason for anxiety comes up: How much time do I have to finish this one before I should start on the next? I'm so burnt out, my concentration is slipping, but I still have to keep going, and get these done, even if it isn't coming together. That doesn't matter, these have to get done somehow! That contributes to my anxiety most of all.
I have reduced my anxiety somewhat by taking a slightly different approach. When I started doing backtracks, I would transcribe instruments one-by-one, and only add instruments when I had to. But now, when I start a new track, I listen to it carefully several times, trying to broadly take everything in. Then, I lay all the instruments out ahead of time which I think I will need. I used to avoid this strategy like the plague when I first started, because I thought it would overwhelm me more. But actually, laying the tracks out ahead of time helps me, because it forces me to look at the big picture, which in a sense is something I am always afraid of doing. But by looking at the big picture right away, I can start to break it down before I put down a single note. I can mull over things like "How many guitars are there?" or "How difficult will it be for me to pick out what the strings are doing?" And by listening to the song a few times after making a template in my head, I can then ask myself "Will this template work for the whole song or will I have to account for odd things that pop up here and there?" My goal during this process is not to make any hard commitments. I might still end up changing a lot of things I initially set up a long time ago, but at least I have a mental musical outline right away, and I can just tweak that as needed. I can anticipate difficult spots earlier on.
This only tackles one part of my anxiety though. It doesn't solve the problem of knowing that hours of hard work will pay off. It doesn't help me keep a healthy pace when working on a set of tracks. Dealing with my lack of concentration and procrastination is a totally different can of worms which I will have to tackle at some point. I keep seesawing between hating myself for procrastinating, and cowering in terror because I am so afraid of starting something I can't finish. And that seesawing is what makes me procrastinate as bad as I do.