I think my obsessions are the only things that keep me busy throughout the day... and to some extent are the only thing that keep me sane. Any "obsession" of mine can and will likely cause me to shift my focus to said preoccupation and take away any focus of the more pressing matters. I've ended up taking bathroom breaks at jobs just for the sake of having some alone time to contemplate some new ideas had for any of my interests I was busy with at the time.
Though I guess, there's also the entire thing where I believe that any obsession or interest I have does contribute to my overall mental well-being a lot more than these "forced" things that come with life. I rather spend 20 hours a day obsessing over a new piece of music I wrote rather than focus on a job at hand. Any idea that will come up is an idea wasted if not written down or executed. Guess that's a bit of a lack of impulse control as well. Though knowing myself, this kind of "acting on impulses" is the only way I get something done at all. I live on the impulses of obsessions rather than the more common notion of "making time" for it. What's also important (at least to me) is that I know that I have the time and resources to obsess over something. Nothing puts a damper on doing anything more than being told you cannot get busy with something (for whatever amount of time you want to)
Most of my obsessions can be tied together somehow, even if it's by a big common denominator like "art" or "self-expression". Sometimes I'll have an extreme focus on something, but it's rarely for something "new". The best my obsession can shift around is that I might not be interested something with a certain detail, or of course, I might be extra interested because of a certain detail. Example; I tend to paint mostly sci-fi oriented miniature figures, yet I've also ended up taking a break and went for a more "high fantasy" approach and painted knights and horses. I do find that anything I do at that time does not involve sci-fi (as much as I love the genre), including getting into specific genre movies, comics, games. I think I just need to feed into a certain mindset to keep it active and interesting for me.
Finding an obsession seems a bit forced to me. I did however find that sparking interest in something (that might eventually pick up as an obsession) often went with having a project of sorts where I needed this kind of skill, mindset, interest, in something. I've had a short period where I got really into victorian era sci-fi (and everything related to portrayal of sci-fi in that era), though that was mostly because I was working (and technically still am) on a project that's kinda related to that. So my interest in that came from research I was doing for a personal project. And similarly, for that same project, my miniature painting interest picked up again (after a 2+ year break) , cause I was painting a prop for an illustration.
With the examples you mention; do you have any intention of doing something when you're able to play the guitar? Like record music? Play in a band? Go solo? Or is it just an interest you wanted to get into to, just to kill some time? Cause if it's the latter, no wonder it doesn't stick (at least, to me it makes sense it doesn't). I need intrinsic motivation do get something done and if I'm doing something just to kill time or to fight boredom, it'll be counter-productive. Learning German you say? What for?
I think having interests does need a foundation why you're doing it. And I personally never found the entire "because I think it's fun" to be the most solid argument I can trick my mind into for motivational purposes.
Every now and then I do however get at that point where I'm in dire need of something new. That's where my brain goes into this mode where I'll see if I can incorporate something new within a project I'm working on at the time. Getting stuck in this bland rut with a lack of motivation for any preoccupation can be quite crippling, so it's best to keep moving forward.
Lastly, I'll leave this here; "if it's not worth obsessing over, it's not worth doing at all". That's something I tend to live by.
