Off to school leaving mom home to cry...

This is the first year my little girl at 3yrs old has gone to school to be in the special needs preschool. She's only at school half day but to a stay-home mommy it seems like she will be there for hours. Its been the holiday break for 2 weeks and we had just put her in school the week befor break so I'm so use to her being here. Being 7 months pregnant with my second child, another girl, I know that it won't be long till I have another child to spend my mornings with. However as i put her on the bus, this morning tears rolled down my cheecks softly as I waved goodbye and I knew right then that even though I'm pregnant with her little sister she will always be my baby. I just wonder how I transition my feelings to know she's getting to be a big girl and she needs to go to school when I'm so use to having her home with me? What is a mom to do and how will I react in the future when she leaves to college, gets her own place, and gets married because I feel like I'll never be ready to let my little girl go.