I've got about 80 songs "in progress" right now.

After I start with an idea (chord progression, riff, etc.), I might play around with it a while, decide I like it enough to "keep" it as a song project, then I move on to something else. Months later, I might be browsing over my old stuff looking for inspiration, find one that jumps out at me, and start fleshing it out a little more. Then get bored with it, move on to something else, maybe I'll find it a year later and do a bit more. In this way I'm very slowly making progress on a lot of songs, but not making fast progress on any.

I'm wondering if this is a common way to work, or if I have a serious ADHD issue that I need to address.

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I think it is common, but not very beneficial. It is much better to start a song, and just finish it. Otherwise you have a ton of unfinished songs.

But don't worry, I have hundreds like that also. You're not alone.
I do have a ton of unfinished songs, but I think it's important to know when a song just isn't working. You have to be willing to cannibalize riffs and melodies, even lyrics from older songs. You say you "flesh out" an old song, which is good, but I like to look for parts to add to a song that I'm working on now and might be having trouble with. Occasionally a song for me is coming along so slowly that I must abandon it, at least for a while, so don't feel bad about keeping old songs as long as there's some kind of reason to keep it.
I like to record and release music in EP format so I'll generally just have 3-4 songs on the go at a time. There are still lots of songs that I've partly written or developed to a point and then shelved but once I've picked the songs for an EP, I usually won't work on anything else until they're all completed.

That's not necessarily a more efficient method though. My current project has taken around 18 months to complete and that's only 3 songs.
Quote by MrFixIt
None. One must finish what it's been started.

this is the correct philosophy. But sadly one I frequently fail at.

I have so many recordings of licks and riffs, and sometimes chorus and verse with a melody, and nothing else, that I cannot go through it all if I want to write something. I just find it easier to start something new. I prefer to start something new as well, because it feels more like forward progress and is more interesting to me, but every once in a while, I will come across an old recording like that, and get excited by it, and turn it into a song.

I find it is even difficult to write a song while you are doing something else, like learning a piece, or practicing dexterity, which is why I like to go through phases. But sometimes, like right now, I somehow get caught up in multiple things at once. I've got 3 on the go right now.
While I agree my 80+ songs is TOO much, I tend to think one or even 3-4 songs would be too little for me.

It's hard to work on certain songs when my emotions are not in tune with it. So I like the fact I have songs that are peppy, or dreamy, or angry, or sad, or angst-ridden, or funky, etc. It means I can work on whatever suits me that day. This doesn't justify having 80 songs in the works, but maybe 10 would be ideal. if I only had three or four songs to work on, I feel like there would be a lot of times I'd be forcing myself to work on a song when I was not really in sync with the song's mood.

I spent hours over the weekend playing through my 80+ songs, rating them with 1 to 4 stars (or zero if they were crap) and then putting them in a list sorted from four star to crap, as a first step to picking 10 (maybe less) to focus on. I didn't finish, have about 20 left to rate that are transcribed, but then I still have some recordings of jam sessions that I liked enough to save, have to listen to those and figure out if they are worth transcribing and turning into projects.

I think I'm a bit OCD in my need to have the entire world of my potential song projects summarized, listed, organized, before I pick those I want to finish up. Heck, I could just pick some of the best of what I've already rated, but then who knows what forgotten gems might be lurking in my old recordings or transcriptions? I just can't work that way. Damn OCD!
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Depends, sometimes if I feel the riff, melody, or whatever I came up with is decent, I'll just keep playing with the mindset that I wrote the original riff in, and eventually finish it. However, if I write something that sounds cool, but I don't really know where to move with it, I'll store it until I can find a way to add onto it, or I might use it as a "bridge" to a gap in a different project of mine. I don't have tons of unfinished projects, I just have a few ideas here and there.
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I have days where I suddenly have heaps of inspiration and write 2-3 songs all about the same thing more or less. Then other days I just cant do it. I have no inspiration at all. So yeah. I usually have about 2-3 songs at a time. But other times I'll write something I am proud off and then I go to bring it to my band and edit it. So yeah. Its hard writing songs.
I keep a notebook, and I write down whatever comes to me. Sometimes I'll revisit stuff and expand on it. Usually once you get far enough into a song the rest of it just seems to write itself.

I rarely sit down and write a song from beginning to end; that makes it sound forced and inevitably there'll be a part that I feel like I just stuck in there in order to say I finished it.

That said, my latest EP was written exactly that way; just sitting down and writing songs one after the other and not second-guessing what I've done. I'm mostly happy with it. It's not perfect, but it's good. I'll probably go back to my original method for the next one.