When you have a lot of different projects and not enough motivation, energy and time to work on all of them all the time, it's very important to organize them in such a way, that you can work on them in small increments. There are two patterns that help with that, the Flywheel and the Ratchet.
A flywheel accumulates energy. You can make it spin, and leave for some time, and it will keep on spinning, at least until friction makes it stop. You can store energy in a flywheel, when you have it, and then use it later, when you need it. You could also call it a battery, I suppose.
I always try to organize my projects so that they have a kind of psychological flywheel in them. When I have a lot of motivation and energy, I can "store" it in them, and later draw upon it when it gets low. How do I store the energy? There are many ways. I might link to some inspiring materials, I might write down the ideas, I might create a "done list" (it's like a "todo list", only the other way around, more motivating), I might post some questions online, I might order some cool parts, design a PCB, write a proof of concept program, etc. There are all sorts of small activities I can do while I'm motivated. The point is to always make them leave some kind of trace, so that they can motivate me later on. This may be serendipitous, like finding an old picture or prototype, this can be on-demand, like going through my "done list" or reference links, or it may be timed, like receiving the ordered parts. The point is to always store away energy when I have it.
A ratchet is a mechanism that makes sure that you only move forward, and not backwards. If you use psychological ratchets, you can work on your projects in small increments, and still have them all progressing. The ratchet will prevent them from dissolving over time.
Again, there are many ways to introduce a ratchet. Using version control repositories for your code is one such example. Saving snippets of articles you write, even if they don't make much sense yet. Saving sketches. Keeping photos of the things you are disassembling or assembling. Using bolts instead of glue, and plugs and sockets instead of soldering everything. Having room for keeping all the prototypes. Having enough stock parts to not cannibalize old projects. Putting labels on the PCBs, so that you can figure out what goes where even after you forgot. Commenting your code.
Both of those mechanism also work very well with collaboration. In fact, they let you collaborate "asynchronously", without even having to actively cooperate with your collaborators. They can pick up your flywheels and your ratchets years after you created them, and develop them further independently. All you have to do is to make them public, though that is not always possible with physical things.