I'm going to miss my friend's bachelorette party for a work trip
Why would you miss such an important event?
I got promoted to general manager in my company, I'm the head of 50 people, but do I deserve this much success?
If you're promoted you can set your own schedule at that level. You really should consider going to the party, it's once in a lifetime.
I'm tired. I can't go. I'm setting my own rules.
If you really can't bring yourself to go, I understand.  However, you need to make sure you tell your friend that you won't be able to make it, and that it's because of something important for work.  A little context behind your decision should make them much less upset about you not being able to make it.