I’m an artist as well and can completely relate.
I’m still amazed I managed to get a degree, although I had to do it as a mature student as I dropped out the first time around.
I have many projects I’ve started and never followed through with. I found I could do the work when I was at university because it had to be done. If I didn’t do it, I’d fail. It also helped that I was set briefs.
I struggle now with motivation and procrastination, my problem is actually starting work. So at least you’re getting started, even if you’re not finishing.
The only thing I can suggest is writing a brief, setting yourself a detailed project and a timeframe in which to complete it and stick to it, as if you have a deadline and someone needs it. I’ve seen other people suggesting using briefs from competitions, you don’t have to enter the competition but use it as a way of setting yourself the project and having the deadline, but you don’t have to submit the work if you don’t want to.
Also don’t be too hard on yourself if you’re having a day when you’re just not feeling the work. Take time to do something else and then come back to it. At uni we were encouraged to go to the library and flick through books and magazines for inspiration. Sometimes you just need to give your mind a break and then you can get right back into it.