How do we constantly motivate ourselves to finish meaningful things first?

The mentality is like this: I know every time I can focus on only one thing. But when I really dig in, I tend to be absorbed into it totally. After I get out of it later, I realize I have missed something else and regretted why I didn't plan better. What even worse is when I start to get digging into one thing, at the start 1~3 minutes, I feel extremely lonely. It's unpleasant mentality which I want to avoid or overcome for good.

Grab a sheet of paper and a pen. Draw a big T on it, but extend some of the vertical line higher than the horizontal, so that it's more like a cross, but the horizontal bar isn't in the center. It should look a bit like this:
††††

Upper left corner, write Absolute Necessities. Upper right corner, write Wants, Not Needs.

What to put in each column? Well, rent (or mortgage) goes in the left-hand column. So does groceries (although some groceries go in the right-side column). Utilities go on the left. If you're in school, then your graduation requirements go on the left, and your electives go on the right. For all of your activities, make a choice. Let's say you were taking Portuguese as one of your activities, and another activity was basket-weaving. If you're working as a Portuguese translator, or you want to be one, then the former should hit the left-side column, yes? But if you want to go into weaving, then the latter activity should hit the left-hand column.

Move things from side to side as you decide on priorities and personal value. Recognize that some things provide a value that isn't on the surface. Maybe your basket-weaving class provides some badly needed relaxation time, or you might even be saving money on gifts by making your own. Maybe you met friends through your Portuguese class, or you want to visit the country to see family or whatever.

Not all value is in money or work. But you also don't have to do every single activity that is presented to you. There are some outright necessities in life that are nonnegotiable. If you have children, then they need to be fed, shod, and cared for. Everyone needs a roof over their head, and they all need medical care. We all need to have clothes on our backs.

But what else do you need? And if something that isn't a need is a bother, you can maybe stop doing it, whatever it is.

There are things, important things, and urgent-important things.

I prioritize constantly.

Meaningful things being meaningful should not require additional weight to be satisfied. Your question thus seams a bit contradictory. I suppose you rather mean to ask how does one reconciles between current meaningful things from things that we find to be more meaningful later on. The answer to that is simple, you negotiate based on your previous experience on what ought to be more meaningful in the long term. So long you are coherent between past and present choices there is lots of choosing possibilities including selfish ones.