"There is no reason for me to feel this way. So I'm not going to feel this way."
Most depressed people can't think that way. Most depressed people can't even imagine it possible to just say "I'm not going to feel this way" and have it actually be within the realm of possibility. You're not most people. You might not think so, but I think you're stronger and more determined than most people.
Don't confuse a sad moment, a sad day, or even a sad week with depression. Sometimes it's just a sad moment, a sad day or a sad week. During tough times with marriage, children, work, when life seems hard and I feel very tired, I've thought how easy it would be to just turn the car a bit to the left and it would all be over. But I don't consider myself depressed or suicidal. It's just exhaustion and at those moments, the good parts of life don't seem worth all the effort. I think we all have moments like that. We just get tired and it all seems pointless. I know that when I'm stressed and have a lack of sleep, is when I have those moments. You have just had a baby, are not getting enough sleep, adjusting to having two kids, taking on other people's kids, trying to keep your marriage happy, your home in some kind of order whilst still pretending to the world that you're superwoman with a big smile on your face.
Create a life that works for you, not a life that you have to work around.
You have lots of friends that love you and want to support you. Doesn't that say something about who you are and the difference you make? I think that alone makes your life worthwhile.