Comparing children

Mandi 😈💀 • Expecting #2 August '15!
I'm having an extremely hard time not being sad an upset by the differences in my children. My son was/is definitely behind the curve, he has a major speech delay, and did almost everything late. He's a smart and happy boy, and he has an early learning interventionist who is helping him, but as my daughter is hitting milestones I am realizing just how far behind he was on things I didn't even notice. She may also be ahead a little bit, but I'm just realizing how clueless I was. 
I'm not sad that he's behind, he's getting help and progressing wonderfully and that's what matters to me, but I'm sad and upset at myself that I couldn't see he was quite so far behind. He didn't smile until he was 3 and a half months old, she smiled at 4 weeks! And even once he did start smiling he did it very rarely at first. With her it's like one she started she smiles all the time, and responds to funny faces and baby talk, where he didn't until much much later.
I just don't know how to stop comparing them and being so sad and upset by it. Just needed to get that off my chest. Thanks for reading.