I have no problem with flow if I'm not paying attention to commas. However, once I pay attention to commas, my sentences become shorter in order to avoid making comma errors. What do I do?
I try to cut down on commas wherever I can. I read my stuff aloud, and usually it sounds better if I'm not pausing all the time. And don't worry too much about comma errors. There almost isn't any such thing. The rules are mostly just guidelines and if they don't work for you, don't follow them. Write your sentences the way you want to, the way they make sense to you - the way they sound good when read aloud!
There is nothing wrong with short sentences. If you choose to use a compound sentence, make sure it is appropriate to combine the clauses in a single sentence. One sentence should express one idea. You can modulate flow. Choose longer sentences for a leisurely, flowing pace, but retain the principle of one idea per sentence. You can use description when the story is rolling along between action sequences, and d use the slower pace to accentuate the subsequent brisk action scenes Shorter sentences add punch. They amp up the pace. Leave out unnecessary description. Keep it moving. Keep it tight.