What should I say?

Tell your mom that you are afraid of this transition and that you want some help trying to find comfort with it. You don't want to keep that sort of thing balled up inside because then she will never know what you feel and when things or if things get more serious this will just add on to any problems that you could possibly have in the future. Just have an honest chat with her.

I understand that might be uncomfortable for you. You're so used to having your mom around... and suddenly there's this strange guy who's always there. It's different. My mom kinda went through the same thing, and for her... she was just really excited. Think about how you are when you have feelings for someone... it's exhilarating. She's had you and your brother for so long and you probably mean the world to her, but she's been missing another part of her life. She probably is so distracted by this that she doesn't even realize how it could make you guys feel. Tell her that you miss her and would like to have some family time... let her know that it's going to take some time to get used to this new guy but you just need to start slow. She won't be able to do anything about it unless she knows.Good luck... Hope this helps!

Cut the ol' gal some slack. Its extremely difficult being a single parent; not simply due to carrying for the children by yourself, but the loneliness of not having an equal, dependable partner to lean on and comfort you when you become unsure of yourself. I know you are going through your own changes, but if your mother has always been someone you could trust, then trust that she is a great judge of character and this man entering your family will only be a benefit to you. Just take it slow, let her have her fun for now, as parents (especially single parents) don't get this opportunity very often. If things feel like they are falling apart due to this relationship, then you can worry, but for now, celebrate your graduation and the possible formation of a more complete family.