Why is my cat so seclusive and isolated?

My cat is very reclusive and hide all day and night. She comes out once in a while but I worry about her. She doesn't get along with any other animals and often growls. She was a partially outdoor cat so I think she has a survivial instinct. I got her as a two month old kitty and she is still and always has been very shy, very quiet and sweet. When I first got her I knew she would be best friends with my other cat and they were. They would hug and cuddle for half of the day or more and constantly groom each other. A few nonths later she ran away and came back six months later knocked up. My first cat sensed that she was pregnant and actually showed sings of jealousy! Since then they will not be friends. Then the second time my Maine Coon ran away she had frost bite on her ears and lost them. One day she looked inthe mirror and became so sad. I know people say cats don't have these kinds of emotions and cognizance but I swear I know what I saw. Anyway it's been four months since she came back from her third trip outside. I know being a Maine Coon she has to be an adventureous outdoor cat but how can I help her adjust?