You do what you can do! The fact that you care and are feeding her is a good start. Maybe if you explain the situation to the vet or find a lost cost spay/neuter clinic you can at least get her spayed. She should be fine set up with lots of straw and shelter in the barn. That way you can keep an eye on her, and she will get plenty of food.
We all understand what your going through. I really think she will be fine outside because she is already used to it. The storage containers that Kim mentioned is what I use and they work great. I stick a big wad of straw in and push my hands into the middle to make a hole. That way the straw is all around the outside areas of the container, like insulation.

Bless you for caring so much. It's heartbreaking isn't it?