Why does watching your kids grow up make YOU sad?

So this weekend has been hard for me, in TWO ways. First of all, I stopped "feeding" Ryker. I know what you all are thinking....how old is he? To answer your question he is 13 1/2 months. I "fed" Hailey until she was this old also. Mike has told me the Ryker has been "recreational eating" for awhile now, but I have had a hard time because he is my last baby. So....bring on the soreness for a week, and FINALLY I can wear a tight sports bra, so that I don't worry about losing my milk!!!
Hailey is now a SUNBEAM! I know this is all normal and Hailey NEEDS to grow up, but last Sunday when I dropped her off and picked her up from her LAST nursery class, it was soooo sad. Hailey turned to me on the way out to the car, "don't be sad mom, I'm happy." I have had the hardest time with this.....She has had many people call and ask me if she can come over and play with them, and I tell their parents NO, because I have a hard time dropping her off, and WONDERING the whole time she is gone, if.."If she is okay," "If she is crying," "If she is sharing, or if her friends are sharing," "If she needs to go potty and feels comfortable going at the house," the list goes on and on, believe me it does, and I don't know if this is normal. I feel so bad, I know she needs this social part of her life, but how do I get passed it?
Here are some updates:
Ryker is now pushing everything we own out of his way, he is WALKING WITH everything. It is sooo funny. He says, "ot,oh" and "mo" meaning more, and for those of you who remember Haileys' little water noise, yes Ryker does that as well, so funny. We are working with him doing the stairs, and practicing "feet first", he is doing really well. Hailey is doing great with letters, she now knows the upper and lower case letters, still getting b and d confused, but really who can blame her? She is doing sooo well with numbers too. We read letters all of the time, and now have started sounding out letters together. She knows t, p, k, n, b, r,w,m,h,e,g,l,o,s,y,and we will be working on others soon. He and Hailey are really doing better, they are not coughing anymore and they are sleeping great.
They play with each other now, and that is soooo nice. It has been a hard year, of teaching Hailey to play nice with him, and teaching ME how to be patient, and kind towards them ALL of the time. They really get along great now, and I think the stars are aligned for at least this moment in time.
So off to meet the SUNBEAM teacher, I know waaaaaaa JESS, get over it, Hailey is.