I've done this trip a few times. Your only real option is to take the ferry to Playa del Carmen (about an hour, maybe US$12) and then the take a taxi (US$3 or so) to the new ADO bus termina. Then ADO bus to Chetumal (4 1/4 hours, US$16). Then transfer to a Novelo's bus into Corozal Town (1 hour including border crossing, US$2). Your luggage should be no problem as the ferry and bus people can help with it. From there you can fly non-stop to San Pedro (US$35, 25 minutes) or take the Thunderbolt ferry, 1 1/2 hours, US$20).
--Lan Sluder www.belizefirst.com