"You work outside the home. Tell me I'm not going to screw up my son by leaving him every day to go to work. I always wanted to be a SAHM but in this economy there's no way we can make it anymore, even for a few more months, without my paycheck. And I feel lucky to have found a job so fast. It's not amazing, but I can do it and the pay's decent. Just tell me he's not going to be forever damaged because his mom walks out the door every morning and doesn't come back for 9 hours."
Ouch. Ouch ouch ouch. Being a mom hurts in all sorts of ways, big and small, transitory and lasting, and from all sorts of things, doesn't it?
Before we go any further: You are not damaging your son by working. He will thrive, because you are his mother and you come home to him every day.
You didn't say how old he is or what your care situation is, but since your primary focus is on your leaving instead of angst about who you're leaving him with, I'm going to assume his caregivers are trustworthy. As long as good people are caring for him every day, he'll be fine under their care, and ready to be with you when you're home with him.
I wish that we all had jobs we loved and feel are worthy of our time, but earning the money to support our families is honorable and right. You are a good mom. You were a good mom when you were home with him, and now you're a good mom because you work.
Those of us who've felt guilt at leaving our kids to work, how do we get past it? Is there anything we can do to make it feel right for ourselves?