I find myself devoting enormous amounts of time and energy into starting stories, but my plots aren't solid and eventually, the story refuses to go anywhere. Are there any exercises that can really get my creative juices flowing?
Advice I've heard for this: identify and strengthen the main conflict, raise the stakes, brainstorm different possible resolutions for this conflict, think up of an ending and write backwards until it meets with the story you wrote so far, even... forget about it. Work on something else, until your subconscious thinks up of something, because maybe all that time and energy is stifling the intuitive plotter in you. I've recently read the advice that writers should avoid "bathtub stories" where a character sits in a tub or other confined space, ponders about all sorts of things, but never gets out until the end. Maybe it's having started off in fanfiction, where getting another character's voice spot-on or with a twist in interpretation or even just bringing a setting alive in text is already an accomplishment, no plot needed, but I love reading vignettes. Surrendering, sometimes, to writing pretty plotless pieces -- I think that's a viable option as well Hope this helped.
Pick a character and a goal, and then start designing obstacles that prevent the character from reaching the goal. Now find a way for the character to attain the goal anyway. Then make the obstacles more daunting, and repeat.
Why ask for "plot creation exercises" if you're probably going to just end up creating a lot of plot ideas, then abandoning them when they stall out...? It sounds like you already HAVE ideas. They just aren't strong enough to sustain a story. You might just need to think over your story ideas/plots longer. If you start a story when you have a beginning in mind, but you haven't figured out at least the basics of where it's going to go and how it's going to get there, of course it's going to stall out. You can't drive a car without gas. The next time you have an urge to start a story, instead of starting it, sit down and think it over for a while. Mull over the characters, what they're doing, why, and where they're going to go from here. You don't have to plot the entire thing out but you should at least have some ideas in mind (aside from how it begins) before setting out. In short, fill up the tank of gas before you go traveling, or you'll stall out. I don't outline my stories, but I think over them a good long time (up to several years) before starting them. If I didn't, they'd go nowhere.