Oh wow! Well, it's been awhile for me. I'm a mom of four myself now. But I'm on the other end of the spectrum. My second is starting Kindergarten in the "big school" and my youngest is going off to preschool in the Fall (wahhh!!!). No, really, I know it's kind of a wacky comparison, but from the parent POV, it's the best I've got. I was a much bigger ball of emotions when my first dd hit major milestones than my subsequent children. It doesn't mean I didn't celebrate them and feel that tug of, "My how time is flying." It just means that I have come to learn that we are adaptable and capable of adusting to the normal transitions of life.
For your mom, are there any hobbies or activities that she loves to do that she hasn't had the time to do as much lately? Maybe you could encourage her to pick that back up. I know this may sound hokey, but part of you going to college not only makes her sad to miss you but it makes her face her own mortality too. Times a'flyin'. Her baby that is seems she just brought home from the hospital is leaving the nest.... ALREADY??!!! My, where has the time gone? So, anything that reminds her how wonderful, talented and how much she still has to look forward to in her own life would be helpful. It's so hard for a mother to have this empty "hole" where she used to put so much time, worry and attention. At least she had your physical being there to touch base with. Now, for the first several weeks at least, she'll still worry if you get enough to eat, enough sleep, are happy, are studying, are making the right choices, but she won't be able to physically look you in the eye and touch base with you. Then the realization will slowly set in that YOU are in charge of you now. She will slowly need to let go of worrying about how well you are eating or getting along with your friends or if you need a new pair of sneakers. That's tought stuff. What do you fill that part of your brain with? She'll figure it out though!
Can your mom and you put together a scrapbook (or maybe two, one for each of you to keep), about the past 18-19 years of your life? That could be your summer project. If you don't have pictures of certain fond memories, just write about them in each other's scrapbook... kind of like a journal. In a way, you'd be putting all your past memories in place... (you know the saying... closing the book on this chapter of your life?).
And once you are AT school... if you have time, you could both agree to just use a regular notebook and journal in it a few minutes each day. Then once a month you can send your notebooks to each other to read. (Obviously, only write about what you want your mom to know)... but if you do this every day, she'll still get to read about the little things you think, feel or do that you might forget or not think to tell her in phone conversations and such. Plus, if your schedules don't jive to talk on the phone... or suddenly, you're feeling it's not so cool to spend so much time on the phone with mom in front of your roomie... well roomie doesn't have to know the journal gets sent to mom, at the end of the week or month, right? What's cool about this is you'll have some very detailed journals, great memories of your first year "on your own!"
Have faith in the fact that she will adjust. You both will adjust. It's okay to simply say you will miss being a part of the family's daily routine, but that you still love them all very much, they are the core of your life and this is a good thing for you to do for your future. Remind her she raised you well and now you're ready to go out and prove that to the world. Then let your confidence about your new adventure shine. Don't apologize for going to college. It's a great, wonderful thing.
And remember, all freshmen are new and looking to make friends... so if the friend factor has you worried, know that everyone else is in the same situation as you. Even if there are people you know from H.S., or that others know from their H.S.'s most everyone is ready to branch out and make some new friends. So as long as you are open, warm, as friendly as you can be, approachable it should fall into place.