Why oh why am I doing this?
Why yes, it is an excellent idea to attend a joint bachelor/ette party with your recent ex. Of course it is. Because you're going to be "friends." And it's going to be "fun." It makes sense, right? Because he is now a confirmed bachelor. And that makes you a bachelorette. It's like this event was made for you two.
And maybe you have something to prove. Maybe you don't want to have to sit something out because he gets to be the cool one while you cry in a corner. He doesn't get to put you in the corner, right? You don't have to care about what he thinks anymore.
It'll all be okay, so long as you are truly committed to having a wild bachelor/ette time, and promise yourself not to ruin the mood by actually crying in a corner. That corner is out of bounds. No, you are going to have fun and you're going to look good and no one can stop you! You're losing crazy weight because when you break up with someone the smell of food makes you feel sick, and you should really capitalize on this fact and make the most of it.
Besides, you've only been a maudlin drunk that one time. You can handle this, right?