I remember reading this when I went through my breakup. The anger and grief overwhelmed me and had me thinking and acting irrational. This was the most dangerous part of the five stages because I could have lost a friend and someone who will always be important to me because I was venting that frustration and anger at them after the fact. Fortunately, I was able to put everything into perspective very quickly, I apologized for my immature behavior and feel that I went through a tremendous growth period. Today I can say that because I was able to self-reflect and understand I couldn't continue to stay in the Anger and Grief stages, I am in a much better place and have a decent relationship with my ex.
To anyone going through these first two stages, things can and will get better if you let them. Get yourself to Acceptance as quickly as you can and remember how powerful the way you treat your ex can positively or negatively affect any potential friendship but even more importantly, your own self image. Be the best possible version of yourself and you will be rewarded.