Upvote:6
There is (generally speaking) no way around this. In your situation you have four options:
Cancel the original ticket and book a new one. This may be useless if the original fare doesn't allow refunds. However it would free up a seat for some other traveler who might need a last-minute ticket.
Book a new one-way ticket and alter the original reservation. You've already found out this is prohibitively expensive, since one-way fares can often be more expensive than round-trip fares. The reasons for this are discussed here.
Alter the date of the original Brussels->NY flight so that you can get there earlier.
Ignore the original reservation and get a brand new return ticket.
The airlines are following a certain contract to which you've agreed when booking the ticket. It may or may not be absurd, but that's just how modern airlines operate.