I saw that you wrote a few comments along these lines, that you can establish and beginning and end to a story, but the middle tends to get endlessly long, daunting and complex.
One thing that I'll recommend is to maybe start out with some short stories/one-shot comics first The idea there is that you set yourself a page goal/limit, like maybe you want it to be a maximum of 50 pagesor something (or 20 or 30 or wev), and then use that size limitation to build your comic within. It's a pretty helpful writing exercise because it forces you to really carefully consider what goes in the middle, and how to most effectively get from the beginning to the end. If you spend too many pages on the middle stuff, you'll run out of space to draw the climax and ending scenes. So you have to pick and choose the most important scenes to connect the dots, as well as how long you can afford them to be in order to complete the story and maintain good pacing.
If it sounds hard, that's because it pretty much is xD I'm in the midst of working on my first webcomic currently, which I'm looking to be a 40-ish page one-shot. I've already gotten to a couple of scenes where when I initially write them out, they're these long, cool action sequences, but then I realized that for just a random mostly unimportant scene in the middle of my story, they were way too long. So I had to edit them down a lot to serve their purpose still, but be concise and to the point: there was plenty of time for action during the climax.
Lastly, one thing you can do is take some of your current story ideas and turn them into short stories like this. You can use some of the same characters, locations, and story ideas so you don't have to think of completely new ideas. One approach is to try and focus in on the first chapter or arc, and really flesh that out as a stand alone story. Or perhaps you write a new scenario in one of your worlds. Either way, it leaves the door open to later either expand on the story that you started, or start anew with a longer tale in mind (where you have more space to expand on less important events. If I was writing my comic as a longer story, I probably would have kept that long fight scene as is, and expanded upon it more, turning it into its own chapter, for example).
A little long winded, but I hope this was useful xD Granted, you don't have to start with short stories, but they might be a good writing exercise to practice writing "middles"!