AIO for not being invited to my mother's proposal even though I designed the ring

My mom (38F) and her bf (40M) and about to get engaged, unbeknownst to her obviously. My mom has always been CONSUMED by the idea of being a wife, no idea why, and this will be my mom's third marriage. She has been with said boyfriend for 4 years and she has asked me several times if he has approached me with the idea of proposing and I always just tell her, "Can't tell you that" because it will ruin the surprise. My brother, family and I aren't the biggest fan of him and although I try my hardest, I am a very stubborn headed person and can't see eye to eye with him really AT ALL and since I still live at home, we butt heads way more than we should.

When he asked me if I wanted to come with him to look at rings, I obliged simply for the fact of being respectful and because I want my mom to have only the best (I don't trust his judgement). We looked around town, but most rings were either really pricy or just not the best or what she would want. He ended up finding a website where you create the ring from scratch. He sent it to me wanting me to design what I think she would want. So, I literally chose the setting, style, and cut of diamond.

The ring showed up 3 days ago and he called me because he was freaking out because he had a plan to do it New Years Eve which is their anniversary, but it showed up late, so he doesn't know what to do now. I suggested Valentines Day or even picking a nice date in the spring and he mentioned possibly doing it during their trip to Hawaii with their friends in April. A trip I am not invited to. I simply said, "oh well I was kind of hoping to watch it happen" and his response was... a cold as fuck, "it's not your day". I even tried to compromise asking him if he'd at least have her friend facetime me and he said, "I don't know". At this point I was stunned because I understand this day is not about me, and I would never try and make it about me, but this man had me design the ring and he's trying to blend families with us so I figured the least he could do is let us (me and my brother) be there for our mom. My mom means everything to me because since my dad left, it was just her and I for a long time before she met my stepdad and had my brother. Her happiness is my happiness and I just want to be there to witness the look on her face and how happy it makes her.

I have my grandma telling me it should be something intimate between just them, but I know if roles were reversed my mom would be PISSED if my bf said no to her. Obviously whatever happens is going to happen, but I just feel kind of used and tossed aside. Reddit how do I approach the situation?