A few rapid fire ideas.

Don't rule out medication. It can be the thing that lets you get over the first hurdle so you see what it's like on the other side.

Go for variety. I'm a software engineer but I am doing very physical and mechanical things in the evenings, at least until winter. Then I can do some tech shit

Be intentional with your time and resources. Think about what benefits the journey of your project will have for your personal well being. Being conscious of the benefits of an activity lowers the the threshold for the effect of "just push through and then you get used to working on it and keep going and now it's your hyper focus."

Own your decisions and keep them to yourself at first if you have to. If it's a hobby you're interested in, you will probably do a good job learning and you will produce good results, and that will help build the kind of deep fundamental confidence that you can't just decide to have.

I am very much an ADHD + Rejection Sensitive Dysphoria type. There's a related issue called Avoidant Personality Disorder. I have gone nuts on my conduction projects for my hobbies without constantly including family and friends like is typical in my circles when doing big yard projects.

So while my advice inevitably will not work for everybody -- we're all different and we have to reverse engineer our own brain & body manuals -- hopefully some of this will make sense in this case.