I'm the same, and can't spell my way out of a paper bag. Also, due to the diversity of my reading sources as a youngster, I've found after posting stories online, that many of my 'American' spellings are actually British. Since they're for uncommon terms, I'd never realized I favored some British spellings over their Americanized versions.
Rather than relying on spell-checkers, which are notoriously unreliable, I instead resort to the Google Search bar in most internet browsers. Whey you type in a word or phrase, it 'guesses' what you actually wanted to say, rather than 'correcting' an incorrect word, so you can often spot the correct spelling for the word you're searching for.
If it wasn't for Google Search, I'd never have become an author!
Also, though Ernest's suggestion is the 'official' solution, it's also notoriously unreliable, as the editor lists are often outdated, the authors don't respond, or when they do, there's a clear difference in interest based on the story contents.
Better suggestions are to either ask for editors here, or to study the authors you favor, the ones writing similar stories to those you want to write, and to identify the editors they credit. You can then either reach out to those editors yourself, if they're also authors in their own right, or you can ask the authors whether they could recommend editors for you.
But in the end, the best bet is after you've already posted a story, and readers write in to point out mistakes. Pay close attention to those, as the majority of my best editors have come from those very reader comments.
I'll often vet their corrections against my other editors, to ensure they know what they're talking about, but if they consistently offer decent suggestions, grab them by the lapels and beg them to help!
The other key, is you don't just ask for 'an editor'. Instead, you want to give them some context before asking them to commit.
It's best to mention the story, include the story blurb (make sure you have that formalized before you ever write anything!), and include the genre and any relevant SOL codes and how much sex it contains.
That'll prevent you getting someone involved who'll only decide to dump you shortly afterwards. The beauty of readers who volunteer is that they already know and appreciate your storytelling techniques, so they won't try to modify your writing style completely.
Best of luck finding someone. You'll find that, as much as we fight and bicker with one another here on the forum, we're all working together to help new authors find their way through their first story.