I'm so gald you came here and posted. It wasnt only very brave but I think we can help you.
I'm 66 yo, I've got a daughter that has had 2 step mums. I've had one 10 year relationship that I was a step dad for her two children. I've had lots of problems and had to overcome them with step parenting.
The first thing I noticed and it sticks out a lot, is that you claim you said- "I dont know him well". I can say from lots of life experiences that many times I felt I dont like someone then once I get to know them, I find I grow to like them. Your feelings could be influenced with some level of jealousy in that suddenly you realise you have to "share" your mum. This is also very normal for your own feelings. It isnt unusual at all. Dont feel like a fool, I felt the same at 40 years old!
Some thoughts can also be unrealistic. Like your fear of "abandonment". Your mum isnt going to leave you. That is the jealousy playing up again. If your thoughts are fake then try to discount them because your emotions react to your thoughts and all of a sudden its sadness and panic.
I think your mum is reacting in a hostile way because she feels she needs and want the love of a boyfriend but feels your fear and apprehension. She would love for you to think of her needs and wish her well, give her your blessing. A big part of maturing is to think of other people before you think of your own feelings. I think that is your challenge, to portray some grown up attitudes by telling your mum that you have had fear of abandonment and thats why you have not given her your blessing. At the same time tell her you want her to be happy and that you hope to grow to like this man that also want love and companionship.
All of this is why others have sided with your mum. But all is not lost, you can fix this quite easily in fact.
I say "easily" but for you it is a challenge. Firstly, ask your mum if you can have a chat alone, make sure its in a quiet place uninterrupted. Say to her things like "I'm sorry for being an obstruction to you and your new love in your life, I've been possessive because of fear, I dont know him well at all and have prejudged him. I want you to be happy mum and I need help to get to know him better, I want to give him an opportunity to get to know me as well, but I do love you mum and worry your heart will be broken".
In fact if you choose to you can even show her this post so she can read it.