I'm going to miss my girlfriend's (now ex girlfriend) birthday party
Why do you want to miss her party?
After further considerations I think it would be better for me not to attend the party
The fact that things didn't go over well from one missed party implies that the relationship wasn't going so well in the first place. Perhaps it is best that things turned out this way for both of your sakes.
I'll create a lie that will help me get out of it.
I think that it's more wise to just be honest about your feelings and not go. How would your ex feel if you were lying, she may think you were lying throughout the relationship. Why not just show up for one last happy memory before you break up?