Telling friend

Malak
So I'm very grateful and currently pregnant eleven weeks with my first baby. We've told close friends and family expect for one...
She considers me one of her closest friends, and I did at some point but she broke my trust many times. We message all the time (don't live in the same country) and I believe she tells me everything, but I just cannot get myself to telling her that I am pregnant. 
I've kept my distance from her in terms of opening up. I just know I can't hide this from her and she would be upset if I waited too long to tell her. 
I just don't understand why it's so hard for me to tell her. She's tried to be a good friend and is. 
She's also been the kind of friend who is always miserable. When I was miserable it was great but I've moved on and so has everyone around her. I know she'll be happy and supportive, but part of me knows she is bitter about her own life. 
Am I overthinking? I just need some suggestion of how I could tell her and why do I feel this way? She is a friend!