Rescue dog afraid of going outside

Question
I adopted a rescue dog about 2 months ago.  for the first ~2 weeks, she was ok with walking outside.  Unfamiliar people, dogs, and noises (even sirens) did not bother her.. in fact she was very curious.

Then slowly she became afraid of being outside to the point where she wouldn't even relieve herself.  Now we're taking her out on our backporch (live in a city apt) and are slowly working on getting her comfortable again with going outside.

I'm curious if you've ever worked with or heard of a similar situation and if you have any advice.

We've gotten a trainer to come by twice now and that has been very helpful.  We've also just started obedience classes (been to only one) - we have to pick up our dog to get her out and into the car.  She was ok for the first class which was a relief.

At this point, I think it may be a case where it's just going to take a long time to get her readjusted but I'm curious if you have any advice/thoughts.

Thanks in advance!

Answer
Probably one of the best ways for a dog to overcome fear is to use rewards,like small pieces of chopped up hotdogs to try and encourge her,even if it's in small amounts,and even if all she does is just put two feet outside,it's a start,and these things come in small steps