Grammar. The answer is grammar.
Done correctly, you should never even notice grammar exists. It should be invisible.
Bad grammar is distracting. Distracted readers get annoyed. Annoyed readers stop reading.
Grammar is a technical skill, a mechanical aspect to an artistic endeavor. It can be mastered; therefore, it should be mastered if you're serious about progressing as a writer.
Are there writers who never master grammar? Yes. Yes, there are. Your alternatives are the following:
- Pay someone to fix it all for you.
- Expect your beta readers to fix it for you. (Unlikely. It's a huge chore.)
- Keep going and hope someday that it all sinks in through osmosis, aka "reading."
Mastering Grammar the Low-Pressure Way
- Take five pages from your most recent work.
- Ask someone who is good at grammar (teacher or nitpicky online person) to look at them and mark where you go wrong grammatically.
- Make a list of the things you don't do correctly.
- Pick one thing and research it until you understand it.
- Write with that grammatical rule in mind until the right way becomes a habit.
- Move to the next item on the list.
Why This Is a Good Thing
First, when you ask someone to read over your manuscript, they'll be able to focus on the art instead of the mechanics. They'll give feedback on your work rather than being slowed down by sloppy grammar.
Second, when you break a grammatical rule, it will be intentional rather than accidental. It's the difference between looking like you know what you're doing and... not.
Third, you'll have to spend less time editing. Sure, you'll still have typos, but overall your first drafts will be much cleaner and you won't have to go through fixing every single line during revision.
Exceptions
If you're sharing your work for free, readers are much more lenient and grammar isn't as important. If you plan on publishing for profit, however, bad grammar will do nothing but sabotage you. Contests will throw your work out, traditional publishers will scoff, self-publishing will be less profitable, and readers will be more critical.
Save yourself the headache and master this skill early.