I believe that your fears and sense of loss speak well to how much nursing has meant to you. She is nursing twice a day now, once in the morning and again later when she's ready for a nap. Just two weeks ago, she decided she didn't want to nurse at bedtime. And yes, sometimes I'm sad. I believe that the sense of anxiety and loss you describe is normal and natural, given the intensity and the intimacy of the breastfeeding relationship.
Nursing is a special time of physical, emotional, and spiritual closeness between mother and child.
However, the growing independence manifested through weaning is also normal and natural. If your son is allowed to wean on his own terms, he will move forward with a sense of trust and confidence. Your child's need for you will not diminish as he weans himself. I experienced the intense sadness you describe when I thought about weaning my daughter, Livy. I also wanted to wean to increase my chances of getting pregnant again.