I don't think these are things that make you a bad writer, they're just signs that you need more than one pass at a thing to be satisfied with it -- which most writers do. Making transitions flow naturally is one of the things I have to spend a lot of time on when I'm turning a rough outline into something more detailed. Like, "okay, I've had this scene in my head forever, but seriously how do we get from the previous scene to this one." For most people, that sort of thing doesn't just naturally come to you. Prose writers even have a name for this -- "The Great Swampy Middle" -- which suggests it's a pretty common struggle!
And I'll definitely write sentences that are way too long on my first draft, especially for characters I have fun writing. The first step of turning my script into a page is to make a sketch of where all the panels will go, and then type out the dialogue in the size I'm gonna use on my finished pages, so I can see what lines fit and what lines are wayyyyy too long and need to be cut down.
These struggles don't mean you can't be a good writer! ;u; These are struggles that almost every writer has, because writing (like art) is a challenge and a lot of work! Most of the time you're not born with a natural ear for things like "how long sentences in a comic should be," you just eventually get that after cutting down an awful lot of too-long sentences. : )
