Ira Glass seems to explain it quite well. Here's my two cents:

Haven't been writing for months. Whenever I say I'm getting back into it, I just abandon it again. Same with visual arts, by the way: whatever I do in Photoshop nowadays comes from an external idea or motivation. It's exciting to get back to it but difficult not to quit after hitting an obstacle.

My guess is - we both think ourselves somehow good artists by default, despite having no education or little practice. Perhaps we're confusing the good taste we possess with skill that has to be claimed through hard work. We think that because we see those amazing images in our heads, we're somehow supposed to be able to reproduce them perfectly - after all, anger is a response to reality being inferior to our expectations.

I used to feel the same about coding, as well: being unable to pull off this or that mechanic usually resulting in pushing the project off, big or small, usually forever. Now... I guess I found out I wasn't a programming genius and am just enjoying doing good. Writing code has always given me pleasure, whether for a personal small library or a big-dreams game. Finding out that I wasn't actually going to be able to work it in a day made me enjoy small steps more.

I have no advice for you, I'm afraid. I'm struggling with it the same as you do. Maybe some of what I wrote will come in handy. Maybe we'll even crack this together in a conversation.