Under the "13 common characteristics of ACoAs" in the stickies at the top of this forum, one of them is that ACoAs are either hyper-responsible or hyper-irresponsible. Difficulty finishing tasks is its own line item, but I think the root cause is the same as the root cause for the hyper-irresponsible.
I'm the hyper-responsible one, and while it's been good for my career, it has, at times, made my life horrible. But I've spent a lot of time thinking about the root cause of the inability to finish tasks or the irresponsible trait. My husband (who comes from a relatively healthy family) has difficulty finishing or starting tasks. He's afraid of failure. If you never finished the thing, you can't be judged on your performance, right? (Yes, the logic is skewed. Isn't most of our logic skewed?)
I do believe that the perpetual putting down and negating of our efforts and achievements is the root cause of the inability to get things done (or the need to be hyper-responsible, which is just the other edge of the same sword - maybe if I do everything they want and do it perfectly, I will get the props I so desperately need).
With the caveat that I don't have my own hyper-responsibility under control entirely (I'm workin' on it still!), have you tried praising yourself when you do complete a task? Any task? It sounds silly, but the idea of re-parenting should work here. You get up, take a shower and head to work - congratulate yourself on getting to work on time, tell yourself how proud you are that you didn't hit snooze 18 times and end up late. If you were late, tell yourself how proud you are that you didn't decide to call in sick, but went to work anyway, knowing you were going to be late. Start with the little things, the things you do daily that you know you can do.
The praise should be in the form of what a parent would say to a child of, say, 8 years old - because usually that's near the age at which your emotional development got stuck. Imagine that you are an adult (which you are) and that the person who performed the action was a child, and praise appropriately. Say it out loud.
I know this sounds totally goofy. I have used this "self-parenting" trick on myself, and goofy as it sounds (and feels, initially) it works. Remember to start small with things you KNOW you can accomplish and things you manage to accomplish every day. Then praise yourself and give that inner child what they were looking for so many years ago - affirmation.
There are no great deeds; only small deeds done with great love. ~Mother Theresa