You're racing much too fast here. The best ideas come from a state of calm, and of flow. They're like being in a butterfly house. Chase the butterflies, and they fly away. But sit still for long enough, and they will flutter down to land on you.
As for how to know if ideas are good enough to stick with; let them percolate for a while. Daydream. Let your mind wander into different storylines, follow around different characters, just... daydream. I do most of my 'writing' in the car or in the shower, because those are places where I can just sink quietly into my imagination. The ideas you keep circling back to, the ones you enjoy diving into the most - these are the ones which, in time, can be fleshed out into bigger stories.
But, in order to reach such a state, you have to slow down. When I knew I wanted to begin a webcomic, but my main Dragonfeathers story was much too big to dive into as my first project, I didn't force myself to think of alternatives. I trusted myself that a suitable tale would make itself known to me, and just kept daydreaming through the various story threads which made up that universe. Eventually, one of my favourites began taking a clearer shape, and that one became Blue Star Rebellion.
I mean, I wrote 90% of my current script within a day of realising "Oh hey, this is the one!" but the process of arriving to that burst-of-inspiration point was very relaxed and unpressured.
Just give yourself time to relax, to allow yourself to stop pursuing ideas. Just play with them for a while. Enjoy the process of exploring, directionless and curious, as the half-formed threads of stories flutter around you. There's no time limit, no deadlines. You can choose to write things down if you wish, but don't pursue a structure. Just dot points of half-formed ideas. You may be in that state for a few months, or a few years, but it's a creatively healing place to be, and every creator needs to spend time there once in a while.