How do you find the motivation to finish?

I'm a serial project-starter but I'm awful at finishing. I'm constantly full of ideas for things to work on/learn about outside of work. I'll start new projects at the weekend and by the following weekend I'll have completely lost interest, having found something else. This is often a cycle.

I feel I could do great things if I could only find the focus to finish them.

This extends beyond "projects" to many of aspects of my life: my job, dating, music etc; I go through short phases of being very excited about something only to feel complete apathy towards it days or weeks later. I've even struggled to find the motivation to finish my CV lately despite having a job I despise.

I've wondered whether there's something about my brain chemistry that causes this (which I appreciate is a very 21st century attitude to have) but I'm curious as to whether anyone else has experienced anything like this? A constant shifting in your interests/motivation to the point where it's impossible to finish (or be really good at) anything. It makes it very difficult to feel I'm using my time well.