Move to an early-stage startup with a highly technical founder so you have lots of hard, no-bullshit deadlines. Basically, put yourself in an environment that forces you to focus. You will then do better work and this will feed back to improve your self esteem and motivation

Also (or instead, if you are not financially/emotionally ready to take the huge leap above), set aside time to practice finishing as a skill independent of project / area of endeavor. I recommend video games with achievements as they have a more objective finish point than most other things you could do for this. Also they are fun and will make you smarter. Start with short, easy ones that have no hidden or notoriously difficult trophies and just focus on building a record of 100% completions. You can then draw on this experience for confidence and determination to help you finish work and personal projects