As a matter of course, not a medical professional:

Double check against ADHD.

If you have it, it changes how you approach all of the advice you might get, because it means your brain is literally physically different in how it manager attention.

If you don't, that's also valuable intel.

Either way, you'll also want to learn what emotions are in play with your new project cycles. Emotions tend to indicate needs, if indirectly, and if whatever need is driving all of the new projects is not met in some way, it'll continue to frustrate attempts to focus on one project. This is because starting a new project is a well-worn path for you at this point. That goes triple of ADHD or other neurophysical forces are in play.