When I talk to people about grammar and language, they overwhelmingly tell me that they're not important because people don't care about them any more. This may be true. But it is also true that to people who DO care--such as knowledgeable readers and PUBLISHERS---they are crucial. A publisher doesn't overlook your terrible grammar and read on anyway to see if the content is good. They'll just toss you on the slush pile, deal done.
IF I start to read a poem and the poet has no grasp of "their, there, they're," I generally move on to the next poem. The same is true of so many other words: its vs it's, lie vs lay, bear vs bare, etc. I've even had to reread poems from the beginning because "bear" being misspelled threw off the meaning of the whole poem. And, beware the homonym--spellchecker can't help you there. Remember that little things that seem inconsequential can add up over time and change our beautiful language completely.
Everyone used to have a good knowledge of grammar in previous eras. Schools taught it, and it was emphasized as a vital part of the curriculum. I have an Amazon Kindle, and I have chosen not to buy many of the self-published books they offer because they are unreadable.
Hire a copy editor, ask a knowledgeable fellow writer for a critique, or join a poetry reading group. It's something small that will make a difference for everyone. And check out The Chicago Manual of Style, or any other well-respected work as a reference. I became a copy editor because I read the Chicago Manual and mentored with a friend. It was well worth it! You may even develop a skill that can earn you some cash in the long run!