How can I be a good closer?
I am better at starting projects than closing projects.


Well, this is not really about you, but the thing in the background is

having an idea for a project is often much simpler than working it out and finalizing it.

Some other words of wisdom are the devil is in the details.

So by declaring that you "are better at starting than at closing" you mount a straw man to distract you from the hard part of research. I agree, the first steps into a new idea are usually fun and some first results are easy to obtain. But, as you also noted, along the way it gets harder and often messier. Going through this, staying focused and sticking to the problem is an integral part of research.

If you would answer with "I could close the project if I wanted to, but I am just better in starting new ones." I'd challenge you to go for it and really finish one project before you start the next one.

As for the question "How can I get better at closing?". One thing is simply to realize that finishing a project/finalizing a manuscript is a hard but important task. Sometimes it feels boring to edit the manuscript once again, go over parameter settings once again, recheck derivations once again, update the bibliography once again... but this is a part of doing research: Bring it in a form that is useful to others.