Coreen:
The good news in all of this is twofold:
1. There are lots of beautiful places in Ambergris Caye at which to have a wedding; and
2. June is not the highest season, so you still have a fighting chance at reservations (someone more knowledgeable correct me if I'm wrong).
Remember this, too: you can have your wedding done independent of wherever you're staying--there are planners on the island. My friends got married at Victoria House (where they were staying), but I believe a lot of it was planned by an outside planner.
Don't panic: your wedding will go great.