Hullo and sorry to hear you are struggling!
One thing that keeps on coming up in your post is that not only are you feeling insecure and fear abandonment - you are mentally beating yourself up over those feelings, which adds to your feelings of anxiety and inferiority.
Take a deep breath. There is nothing evil in having feelings. Really, it IS okay to feel what you are feeling. Jealousy is not something that you are or that defines your emotional existence now and forever more. It is a fleeting emotion - the more you obsess over it, the more power it will gain over you.
You are okay. What you are feeling is okay.
Onwards! There is a lot of help available for dealing with jealousy. If I understood your post correctly, you are afraid that once you allow M the freedom to meet others, she will find someone she likes better than you and abandon you to start dating this new magical person exclusively?
Now why should she do that?
Me: bi female in my twenties