Because the length of a book determines its cost of production and thus its retail price, publishers are extremely reluctant to take on a debut novel that runs more than +/- 100k words. If no one has heard of the author it figures to be really hard to get them to fork out $30 for the hardcover. Agents know this better than anyone. That's why they generally refuse to read a super long ms by an unknown. Time is an agent's primary working capital, one not to be wasted.
I've found that when I make those points in a MS Analysis the writer frequently comes back with the suggestion that the novel be divided at some point, creating two books. Maybe, the argument goes, the agent could look for a two book deal. Or if not, the “second” book could be shopped to other publishers.
My answer is always the same: Can't be done.
No 21st century publisher and hence no agent currently alive and breathing would consider such an arrangement. Here's the unbreakable contemporary rule: EACH BOOK IN A SERIES MUST BE ABLE TO STAND ON ITS OWN, AND TO BE READ OUT OF SEQUENCE. That's important enough to warrant all caps. Moreover, each book in the series must have an entirely satisfactory ending that ties up 99.9% of the plot lines you introduced in the beginning. It's okay—maybe even desirable—to leave a couple of subtle hints about a story yet to come involving some or all of these characters and perhaps their general locale/situation, but if you leave serious unanswered questions your project is doomed.
And yes, I know about TV series that all the time do what I'm saying you cannot do. But that's them and this is us, the folks who deal in words and bookstores (whether bricks and mortar or online). Whether we writers like it or not, readers regularly, buy/read books out of sequence. If what they're reading makes no sense they are unlikely to bother with any other book in the series. Even the first one which would make everything clear.
Given those realities, both problem and solution are obvious. You created these 200k+ words because you believed you needed them to tell your story. Your fix needs to start by examining which plot threads you are actually developing in the second half (or even two thirds) of the ms and go back and take them out. Then you have to figure out how you're going to write a thoroughly satisfying ending to the story involving the characters/situations that remain.
Later, when it's time to create another novel out of the material you eliminated from the first ms you will have to find a way to write a new beginning that feels like exactly that, not a continuation. Then you'll need to find a craftsmen-like way to feed into this second ms that material from the first (hopefully already published!) novel which readers of this new story need to know for everything to make sense. And without revealing so much you kill the sales of your first novel. (And if I may be forgiven a bit of self-promotion: it's situations exactly like this that I address in a MS Analysis and help you to work through.)
That's the long and the short of it. (Sorry, I couldn't resist.) You have to recast the too-long ms not simply divide it. But look at it this way: you've got huge chunks of the novel-writing work done for two (maybe three) new mss. The rest should be…well, not easy, but easier.