I'm not going to an estranged friend's baby shower
Why don't you want to go to the baby shower?
I'm not a good friend and that's the truth.
It's admirable of you to be honest about your flaws.  I think the first step towards becoming a better friend is to learn to be supportive of your friends when they're having big events like this!
I just don't want to go, I'd rather stay home
It seems that you may not be very extroverted and there is nothing wrong with that. However, you should make an attempt to support your friend. Attending a once-in-awhile event and supporting your friend is a good way to try to maintain a friendship.