As others have mentioned, game jams really are your best friend here, though where I see a lot of people make a mistake is thinking that game jams must be an organised event, when in reality you can do your own at any time. You miss some of the fun and feedback of a proper jam, but it's certainly better than restricting yourself to doing it a few times a year (when your schedule and theirs happens to align).
As for resources, unfortunately game design isn't as much of a "studied" field (for lack of a better word) as something like software dev. There's some great YouTube videos and blog entries and even a few good books, but its such a situationally dependant field that you'll need to know how to at least apply your own spin on whatever is recommended to fit your own projects, hence why, for me, game jams are king.
A few other tips I've picked up other the years:
1. Playing games helps, but if you're playing familiar games, you're not going to get much new out of them. You need to go underground, far beyond the safety of the front page of steam (or steam all together). PICO-8 and its splore browser is great for this (incidentally, PICO-8 is also great for game jams too!)
2. Similar to the above, different experiences help you generate different ideas. This can be found through your work by trying new technologies, new ways of doing things, playing with different genres you wouldn't otherwise touch, but also in your personal life, doing things you wouldn't ordinarily do. Just like the above, if you don't change the input you shouldn't expect much of a change in output.
3. Have a little notebook on your desk and anything that pops into your head, write it down, even if it seems absurd. A lot of my breakthroughs haven't come all at once, but rather picking out the best bits from a lot of ideas that otherwise couldn't have stood on their own. In my view, a physical notebook is much better than using your phone or notion or whatever, but I don't really have any solid reason to back that up so idk, ymmv.
4. Not much to say on this one, but keep it simple. It's far better to spend your time making a solid, if simple, base game, than endlessly tacking on features and content to a core that isn't working.
5. Also not gonna say much on this one, because it's a rule I don't follow to my own, very much self-aware, detriment; you need to be disposable with your ideas and plans. Maybe Space Zero just isn't a fun idea, take what you've learnt, drop it and move onto the next project. To be fair, that one can be a double edged sword. New devs do this far too easily, not letting themselves ever finish anything, which is obviously no better.