Just Act Surprised
Whoever planned this surprise party did a pretty poor job of it. The OP knew that she was going to walk into a surprise party, so someone obviously let the cat out of the bag. It might’ve been the scumbag boyfriend, who knew that he was letting her down on her birthday of all days.
You only need to watch that episode of HIMYM once to know that Ted was a total jerk for breaking up with his ex—not once but twice—on her birthday. Well, the same applies to you, jerkwad. We hope the OP could pick up and forget about that guy for good. Oh, and happy belated birthday.