My best advice is to learn to write at shorter length. I'll give you two reasons, one artistic, one commercial.
Commercial: it's getting harder to sell long fantasy or science fiction novels by small-time or first-time authors. It's like mysteries: once a series is established, it generally keeps selling fairly well. It's getting established that's the problem. And until you get to that point, a novel the size of a household appliance, written by an author who only sells a few thousand copies per title, is necessarily going to be a very expensive book. Economies of scale are a major factor in book pricing. High pagecount plus low print run equals high cover price. There's real reader price resistance above $25. When the big chains and distributors see a $27.95 book by an unknown, or by an author whose last book netted 4K copies retail, they cut their orders. That's bad.
Artistic: A storyline is like a rope stretched from here to there: the longer it is, the harder it is to keep it from drooping in the middle. If the point of the story you're telling occurs in some later book, the current book will tend to feel slack.
Or try a completely different simile. Imagine that you're in possession of a 100% clear, reliable, and accurate prophecy which says that eight years from now, you're going to meet the true love of your life, and that it's going to be one of those earthshattering, stars-fall-down loves that leaves everything around you in ruins but is celebrated in song and story forevermore. Got that? Okay. Now imagine that in the meantime, you have another relationship going. It's nice enough. Companionable, even. But on some level, it's got to feel like you're just marking time until the real thing comes along.
I've heard more than one editor describe their sense of faint dismay when they get a cover letter explaining that this manuscript is the first in a series of [howevermany] volumes. A reader standing in front of the bookstore shelves never buys the not-yet-written third or fourth or fifth book in a series. They buy a book now because they believe it will make them happy now. If this book doesn't deliver, they won't buy the later ones.
The book that really set the style for multivolume epics was The Lord of the Rings. There are two things about it you should bear in mind. First, it wasn't written as a trilogy. It was written as a single volume. Tolkien's publisher insisted on splitting it into three books. Second, LOTR's total wordcount is less than some of the fantasy novels sitting on bookstore shelves right now.
Worldbuilding genres naturally tend to run long. Writers and publishers have fallen into the habit of letting their books sprawl. That can't go on forever. If you're bored by overlong books and inconclusive series, you can assume other readers are too. If you want to stand out in the crowd of wanna-be writers, just write the book at hand and make it good. If you succeed, there'll always be room for sequels.
