Whether you have difficulties starting a piece of artwork, or more difficulty knowing when it's finished, can be based in your personality type.
Personality types tend to stay constant over one's life. One can choose to act outside of type, such as performers who are introverted in everyday life, but use extraversion when they're on stage. But we never really change our type. I know this, I thought I wanted to be an extrovert for years. It didn't happen.
It is most liberating to accept one's type, and understand your strengths. We are so much more effective and happy when we are playing to our strengths. I tried this at a party recently. Instead of forcing myself to be outgoing, I sat and had a deep conversation with one person. It was very satisfying to both of us. I enjoyed that so much I'm actually looking forward to my next party!
An introverted artist might enjoy writing a blog or posting photos of his or her work, while an extraverted artist would likely thrive on teaching others and giving demonstrations.
As an artist, a detail person (the sensory type) would love working on the meticulous small steps of stitching or bead embroidery. This would drive a big picture person (intuitive type) nuts! When you have unfinished projects, it may be that once you get to the part that requires attention to detail, you lose interest. You may want to look for types of artwork that can be finished quickly, such as paper casting or randomly painting warp yarns and then quickly weaving them off.
On the other hand, the detail oriented person may have trouble finishing a project, because it can always be tinkered with a bit more. Knowing this about yourself allows you to celebrate your love of detail and give yourself a deadline for stopping work on a particular project.
You can take a quick personality type test online here. Click on Jung Typology Career Indicator Test and you can learn which of 16 types you are. Then a google search on your type will let you learn about your type. We're all something, and when we make the most of who we are, we bring our unique gifts into the world.