I feel upset that my best friend and ex-boyfriend are hanging out
Why are you upset your best friend and your ex hang out? Man, that sucks.
Because I want to avoid my ex but now that means avoiding my friend.
I understand not wanting to be around your ex.  You should tell your friend how you feel about the situation.
I'm just upset. I'm ignoring my friend and sending messages to my ex.
I'm sorry that you have to go through this with your friend. They shouldn't be hanging out with your ex in the first place. But I do feel you should confront your friend about it as well.