I'm going to approach this without thinking that there are any affairs etc. That she's just a friend.
In life, it's wise to choose to surround ourselves with friends who treat us nicely, kindly and with respect. It's OK to dump a friend who treats us badly even if there is history of supporting each other. It's also OK to dump a friend who's simply a pain in the ass.
You don't owe each other anything - and if the friendship is no longer working for you, then just slowly back away. Just as I hope you would in real life if a friendship wasn't working for you.
Edited to add: she has the moods she does because she's a bitch. Nothing more, nothing less.
Never regret anything that has happened in your life. It cannot be changed, forgotten or undone. So, take it as a lesson learned and move on.