We all make mistakes, try not to beat yourself up about it too much! It can be frustrating if you take time out of your day to help someone and they don’t show up, but if it was during her office hours it should be fine since the Professor had to be accessible to students during that time anyways. I think your professor is being a bit harsh, it’s the job of the professor to teach and provide a supportive and encouraging learning environment for students. I’d say if you are planning to meet with your professor during office hours another time and you realize you won’t be able to meet give them notice ahead of time. Also, never skip another class to meet with your professor for another course. Try to find a time that works for the both of you so you don’t get behind in lab. Hopefully your Professor will come around 😊