This has always been a huge problem for me, and many others when it comes to making games. The best thing to do is too just finish one game. Of course, Im not just going to stop there! Theres more too it that can help a lot in the long run!
For starters the first thing you should do is plan out the game as much as you can, and think about exactly how you plan to make certain mechanics, this allows you to figure out things you may struggle with ahead of time, along with things you can do whilst you try to solve a certain problem you may be facing.
You can also schedule your time, I personally found that by scheduling out my time, and figuring out when exactly when to do certain things I became a lot more productive!
I also recommend breaking everything down into smaller goals. For instance what I like to do is when I feel stuck, Ill just go outside and walk around, or just walk around my house and figure out like 3 things at most that I need to do, and then Ill put it into motion. It just feels a lot easier when you think of create kill bricks, checkpoints, etc, instead of create obby.
Sometimes its best to continue to work on a project even when you dont want to, as you will run out of motivation. But also at the same time taking breaks, I used to work on my projects every single day and I got burnt out really quick, now I have set times that I do it, and certain days that I take breaks to just enjoy myself.
As for the idea of working on smaller projects, I used to want to do that, but I found that all smaller projects were really really boring lol 
So I just started working on bigger projects anyways! Sure it was difficult, and tiring, but with the stuff I mentioned before it isnt too difficult.
I also find that working on itty bitty systems to help with future projects can be a game changer! For instance creating a hitbox module. That way on any future projects you already have a base to work from!
Anyhow those are just some of my tips and tricks I’ve gathered over the years, hope it helps even just a little! Good luck!
:D
