Am I being stupid?

I need to ask this because I feel like I’m overreacting. My birthday is tomorrow and I was very excited about it because my mom said I could have a small birthday were I could bring some friends and have fun, that was until she cancelled it last minute and said that she couldn’t do it anymore because she had been invited to a birthday party one the same day as my birthday and later asked if me and her could go. I immediately say no and explain that that party was on the same day as my birthday and that we couldn’t go. To this news she proceeded to say that my birthday wasn’t that important and that the birthday party was for a girls 18th birthday party. After this we had a long fight due to me repeatedly refusing to go because not only was the pasty on birthday but also that she expected me to celebrate my birthday on someone else’s party and be okay with it. Now fast forward to today, my birthday is tomorrow and she is getting food ready for the party and told me directly to my face “ on our way there we need to buy her a gift” with only crushed me event more. I hate sounding ungrateful because I know there are people that have it way worse than me but I just can’t help to think about this. I have one day out of the year that’s about me and yet I’m being forced to sit like an outsider and watch as my own mother celebrates someone else’s and expect me to have a smile on my face as if I haven’t been crying and begging to have this one day to myself. I’m not even excited for my birthday anymore. I know that my relatives aren’t going to say happy birthday, I can’t feel that im getting no gift and I can tell that I’m going to be sing for someone else. I don’t feel like doing anything for my birthday anymore, all the joy I once had left me and now I just want to lay in my bed and cry all day.