I really need some help (step daughter)

My SO has a 5 year old daughter, whom I of course consider my step daughter. She and I have a great relationship, I feel so much love from her and she looks up to me like a mother figure.

From time to time I feel insecure in the sense that she may not see me as a proper parent (my head tries telling me all this bad stuff). My SO always reassures me that she looks up to me like a mother figure and that he also considers me to be a parent to her.

May I just add that in no way am I trying to replace her mother. Her mother is very much in her life. Her mother only lets my SO and I see his daughter every other weekend, so ~26 times a year.

My SO tries to include me in decisions and events such as sports days and holidays, etc. But yesterday evening, I found out that it was my step daughter’s parents evening at her school, and my SO didn’t tell me about it and went by himself. It was him, her mother, and her mother’s partner (the step father). I was the only one who didn’t attend. As soon as my SO showed up, the first thing little one did is ask where I am. She asked my SO to send me pictures of her and her work because she wanted me to see.

It may sound trivial or silly to some of you, but I’m so deeply upset by this. One, that she wanted me to be there and might think I didn’t care enough to go and two, that my SO must not consider me much of a parent if he didn’t even invite me to parents evening. The fact that little one was there with all her parental figures except me is heartbreaking and I feel lost.

I don’t know what to do. How would you ladies feel in this situation? Am I overreacting?

I did speak to my SO and told him how upset I am and how I’m feeling and all he said was that he didn’t think to invite me. I just think that’s a red flag...

I’m just really upset right now.