I think everyone goes through some shade of this problem, sometimes several times. Here's a link to 25 Things You Should Know About Outlining, by Chuck Wendig, who has about a million of these things: (
http://terribleminds.com/ramble/2013/05/14/25-things-you-should-know-about-outlining/)
When you say that outlining doesn't work for you, do you mean the kind you learned in school, with the roman numerals and everything very rigid and formal? Because that is definitely not a method that works for every person (Or every project). But there are a ton of methods that fall in between that kind of outlining and full-tilt pantsing, which also does not appear to be working for you.
I recommend the Snowflake Method to everyone, largely on principal, but you seem like a Tentpole type from your description. Tentpole outlining is a much looser variety, where you only decide on a few concrete moments in the story: Steve gets the Macguffin, Karen betrays him, Steve wins the Macguffin back in a game of high-stakes baccarat. Those are the only things you have in your outline before you grab your machete and head out into the word jungle. Questions like 'How did he get the Macguffin?' or 'How did she betray him?' or 'What the hell is baccarat?' get solved pantser style, when you get there.
As for the concern about "I keep accidentally writing a trilogy with no idea how to write the novels", well, that just means you have to do this four times (Or however many books plus the overarching narrative). The thing is, if there's enough story there, you'll find it. Let's say we expand my story into a trilogy about Steve and Karen. The first book will be How Steve Got The MacGuffin, then The Tale of Karen's Betrayal, and the exciting series finale, Steve Plays Baccarat. Now I need to pick tentpoles for those stories: Steve finds out about the Macguffin, Steve teams up with Karen, The Dangerous Heist Happens. In the second book maybe Steve and Karen fight about what to do with the Macguffin, Steve hides the Macguffin, Karen steals the Macguffin and runs away with the Spoon. Now we just need the parts where Steve tracks down Karen, then her new boyfriend the Spoon beats Steve up, then Steve wins the baccarat game with the Macguffin as his prize.
You just start from the farthest out, whole thing in 20-Words-Or-Less kind of outline, and keep getting closer and more specific until you feel like you've made the gaps between the poles small enough for you to pants from one to the other without getting lost.