I've noticed that it's easier for me to start things than to keep working on existing things, and much easier to do either of those than to finish things. In a sense, this is so obvious as to be boring: of course starting things is easier than finishing them! But, actually, why is that?
In a sense, it's actually pretty counterintuitive. If your life's passion is to fill jars with marbles, the first 50% of a jar is no harder than the last 50%. If you stop halfway through to start a new jar... well, first you'll need to find a new jar, which could be a hassle if the jar shop is closed. And even if you have a new jar all prepared, to fill it you'll need twice as many marbles as to fill the old jar. Even for a marble enthusiast, that's an embarrassingly poor cost/benefit ratio.
I think the first thing we can conclude is that most projects aren't like filling jars with marbles. So, how are they different?
One reason is complexity: for most projects, earlier decisions constrain later ones. Instead of jars of marbles, they're more like crossword puzzles. If you're not sure whether you're getting the words right, things get increasingly untenable as your errors stack up, and eventually it starts to seem like a really good idea to do something that isn't a crossword puzzle.
Another reason is attrition: a fixed chance of abandoning the project that compounds over time. Imagine that, with each marble you put in, you have a 1% chance of breaking the whole glass. In non-marble projects, that might be finding a problem with your approach that makes it unworkable, discovering a better solution, or that the situation that made the project worth doing has changed.
Or something a bit more self-sabotaging, like avoidance: you actually don't want to finish it. Maybe you're worried the thing won't be good enough, or you're afraid of being judged or criticised. It can be more comfortable to toil away on something in obscurity than summon the ego-courage to declare it finished. Who are you to bother the world with this new thing?
But my favourite is entanglement: not finishing something until you finish something else. I want to write something, and I have a project I'm working on, so why not kill two birds with one stone and write about the project! Genius. Except then the project takes longer than I expect and I don't write anything either. Also, have you ever tried to kill two birds with one stone? The physics alone is enough to justify treating that entire analogical lineage with suspicion.
So far we've been mostly looking at root causes of projects being easier to start than finish. Also worth examining are the chronic effects: habit and underdevelopment.
If you tend to not finish things, over time you'll build habits to enable not finishing things. Maybe you'll find coping strategies and workarounds to make it less costly, or develop an almost-sustainable pattern of anxious avoidance leading to last-minute panic excellence. Hypothetically speaking.
And the flip-side is that, even when you try to finish things, you probably don't really know how. Like anything else, finishing things is an ability that has to be developed. It has its own unique rhythm and skillset. I mean, it's not that difficult, but neither is chewing and you made a mess of that the first time you tried it.
It may be better to turn the problem on its head. Perhaps there are a thousand reasons why finishing things is hard, but those difficulties all thrive in the space created by a lack of ability. If you have a lot of practice finishing things; if you are used to the rhythm of it, and have built habits around it; then any factor that makes finishing difficult will have a lot more entrenched structure to push back against it.
So my conclusion is, perhaps unsurprisingly, to practice the art of finishing. Perhaps for me that looks like dusting off old abandoned projects and un-abandoning them, or splitting up large projects into a series of smaller projects that can be individually finished to boost my finish-per-start ratio.
Analysis is often an interesting exercise, but sometimes it's best to fall back on the simple principle that you get better at what you practice.