My high is over 😢

I had my daughter 8/14/19 and when she was born I was just on a high. Like just full of energy, even though I was up feeding every two hours I never felt tired I was still waking up at 5/6am, very happy. But now, 😔 I’m starting to feel down and overwhelmed. I feel drained. I have my older kids who are 6&5 who I feel like I’m neglecting cause I’m so drained that I’m not able to do things with them. Their going back to school next week and I just feel bad.