I can highly recommend Ambergris Caye/San Pedro for a solo vacation where relaxation is the priority. I was there for almost 2 weeks in November by myself. I met a bunch of new people and had me a blast.
I stayed a Banana Beach. There was a lot of construction going on around there, but as I didn't spend much time at the hotel, it didn't bother me much. The hotel is very nice except for the neighboring construction noise.
I know exactly what you mean about hardly knowing you're in a different country. AC allows you to know you're in another country, but still be able to converse in English and get what you need. When I went, I wanted to not see a single McDonalds, KFC, Super 8 or any other American businesses. I succeeded. The only sign from an American company I saw (other than in the store) was a Texaco sign at the boat filling station.
Be sure to wander around San Pedro and visit with the locals.
Nick at Cafe Ole is great. He serves basic but quite good food and is very friendly. I don't believe he's from SP originally, but you can tell he's been there a while.
If you're a diver, I can recommend Ambergris Divers. My last full day there, I was casually walking back to the hotel from town and a couple of the guys from Ambergris Divers happened to drive by in their golf cart. They called out my name and offered me a ride. Just like old friends.
I never dressed with more than shorts, t-shirt and sandals for any dinner while there.
I don't know about being a single woman alone, but I'm a mid-30's single guy and had absolutely no problems being alone. In fact, you will probably meet a group of people to hang out with pretty quick anyway.
The one "problem" with Ambergris Caye is that the beach is not what I call a "pristine white-sand paradise" beach. Sea-grass does build up on the beach although most hotels clean it up daily. You have to go quite a ways out to get into water over your head. Both of these are a result of the barrier reef keeping waves at the beach quite small.
I would not hesitate in going back again and I'm sure I will someday. Relax and enjoy! You'll love it!