Don't be anxious...as long as you are safe/careful, everyone down here is great, very friendly...it will be no problem. I came down here 3x alone before moving here...(now single, 35 year old female)
I recommend Ambergris Caye. It's busy, lots of stuff going on, lots of single people. Mainland is great, but all the places I've stayed are fairly isolated. Stay at Changes In Latitudes. Two women running the place, group breakfasts so you can meet everyone and good suggestions of where to go.
Definitely snorkel at Hol Chan with Lil Alfonse. CIL can set you up. Both Seaduced and Searious are great for tours...
My suggestion is to come here for a week and figure it out from there. Once you arrive it all seems to fall into place.