*Inserts that Lord of the Rings quote as a metaphor.

In recent years I've been playing many different games randomly based on my mood at the time.

But then I eventually came to realize that in this way, I rarely ever finish any game.

So, in order to start finishing some games, I've been trying to change my habits and start playing a single game through to completion, one at a time, whilst not playing other games at the same time.

I haven't been entirely successful with sticking to that plan though, and instead I've been reverting to old habits and playing many games, and also I stop playing certain games randomly, based upon my mood.

I also have come to notice many factors that play into the reasons behind why I don't finish most games nowadays, some of which include: some games might require multiple playthrough to finish all the endings; some games might have only part of the series released om GOG DRM-free and that de-motivates to finish the series-incomplete game(s) that are here on GOG because I subconsciously realize that if I do finish them, then I will be stuck unable to complete the other games in the same series, unless & until they come to GOG, or in some cases, unless & until their console versions are ported to PC in the first place; some games require a massive grind to proceed in the mid-game or end-game portions and I don't feel like slogging through that grind so I just ragequit out the game permanently; sometimes a boss or level is too difficult to complete without first enduring massive amounts of tedious repetition & aggravation & good luck which also might cause me to ragequit permanently, etc. etc.

And another reason why I usually don't finish many games is because I have way too many games consisting of a giant backlog. Many games in my backlog, I don't even know if I'd want to play them even if I did have the time to play them all, which I don't.

So, just the sheer number of games in my backlog makes not want to go through the IRL grind of curating my own backlog in order to select which games I want to play...the very idea of which sometimes causes me not to play any games at all on certain days.

Not only I play only one game at any given time, but also I take the matter one step further, as I most often play all games in entire series back to back (when I have all of them), before I move to something else. For example, Two Worlds 1 and 2, the Secret Files games, or the Broken Sword games (which I’m currently in). The reasons are the same as others stated: to not forget gameplay, hotkeys, story of each. But for me also to not forget the overall story, feeling, and state of the world in a series.

Although I've got much less games than you (as we speak, about 45 unfinished games), what you describe happened to me about mid-October. I honestly couldn't decide what to start.
The solution that came to my mind, and worked, is a plain draw! I put the games in a numbered list and made a draw with random.org! In fact, my current involvement with the Broken Sword series is the result of a draw!
It's good! That way you don't have to think! And you don't expect what you will play. That way you get a result and you stick to it.
Alternatively, you can make the draw only between games of a certain genre, e.g. you have 10 adventures and you draw only between them. Or you can make the draw only between games that you surely most like to play, and one of them gets picked!

I advise this to anyone who is undecided!