Nothing is more detrimental than leaving something unfinished. It wastes all the effort that preceded and puts all the time and devotion to nothing as the next time you are going to start, say after a few days or weeks, or at worse months, then you are going to be completely clueless about what you were doing when you last touched the topic. This is just the nature of human memory, unless you have the blessing of being young and blooming. You will, again, have to start at the beginnning. There will be no better options left for you.
And this makes it all the more challenging. You lose time and momentum. Always starting from zero also bogs one down(motivation loss) and the will to learn slowly keeps fading away on the next iterations is what I have come to realize about this. Sometimes it can happen that you will be thinking that I did this thing completely but you actually only superficially did it, and now you are probabaly just half remembering that you actually did it. And eventually everything stops making sense and you find yourself standing at ground zero.
Obviously there could be certain fields where this analogy may not work and you might be awarded with the fruits of your previous labor. But with many other things and specially witrh coding, you will run into major challenges and obstacles if you leave things unfinished time and again. Your learning debt will be always increasing.
It is also the nature of memory to put off things thatr are dusty anbd not in use. So the less you practice something the more chances are that you will forget about it and the details will become foggy. And if you don't at all touch it, you will forget about it completely in a certain amount of time. Even id you do remember some details of it, there is a high chance that you only remember it just vaguely. You will be all confused if you try to implement the knowledge of the subject in that case. This happens to people all the time in programming.