There is an old saying, "When is an art project complete? When you take it away from the artist."

Personally, I find I have a never ending supply of creative ideas, I just don't always have a schedule leaving energy to indulge in them.

I totally agree with what others have said, join the monthly challenges because they will push you to get things done on a deadline. They are great practice. Plus, you have the community cheering you on so that helps.

Many find that the creative explosions of a project happen in the early phases and the last portion of every project tends to become rendering. Rendering just makes it look just right and takes time. It may take discipline to grind that out. Interestingly the real learning process is in the first phases.
So, if you are just trying to get better at art, it may not be worth the time it takes to render it out to completion. Once you get what you need from the piece you can move on.

If you want to sell stuff, well then obviously you have to suck it up and finish it.

You did mention playing video games and I remember one of my favorite authors answering a question about how he handled playing video games versus writing. He basically said he had to ask himself, "Do I want to be entertained or do I want to be creative?"

That works for me often if all things are even. When I'm bushed after a stressful day I have to be honest and admit it's not in me tonight, I want to be entertained. However, I find the more I ask myself the question, the more I want to be creative.

Another thing you can do is just promise yourself to spend 5, 10, 15 minutes working on a piece. Most of the time you will get sucked in and just keep going. If not, you might get to a point where you finished something just because you picked at it for days on end.