It doesn't sound like there's much to the friendship except that past environment so doesn't really matter if she has "changed" for the better, there's still nothing in common anymore?

Sometimes it's necessary to take care of ourselves. I've dropped courses in school because I couldn't "deal" right then, it's a temporary thing and you may or may not go back (I totally changed what courses I was interested in and never did take the two I dropped but certainly went back to school); write her a kind email and tell her you'll get in touch with her in the future if things get/feel better for you and telling her you really did enjoy her friendship at that time but don't feel you have anything in common and you are elsewhere in your life? Keep the tone pleasant and make it all "your" fault/your "I" statements so she can see it's about who/where you are and not her.

Friendships are not marriage -- they're built for similar hobbies/interests or specific periods of time and you've moved. I have a friend I love dearly (and who loves me) that I met with a group back in the 1980's but we both moved and though we're less than 2 hours away by car we haven't seen one another in 5-10 years and only correspond once or twice a year, usually around Christmas. Things change! It's no ones "fault."