All I can say is that I really feel for you. My daughter just started daycare, she's 1 year old. It was tough being used to spending all day, every day with her, and then suddenly have her spending most of her waking time with strangers. Its only been a month, and I still have weak moments, but she's doing better than I am. There is a 6 week old baby who just started where she is whose mom has just gone back to school.
Remember that moms have gone through what you're going through, and there aren't very many that would tell you their baby didn't LOVE daycare. They flourish there and learn social skills. As your son grows over, no caregiver will ever replace you as his mom. He won't even remember these days when he's older; he will remember who put him to bed every night, read stories to him and parented him at home
You might be doing whats best for the family right now, so just try to think of the positive outcome of you working instead of what you're missing out on.
I have also found that since not being with my daughter 24/7, I've been even more appreciative of the time we have together. I love having all day with her on weekends and enjoying breakfast in the morning and dinner at night, and putting her to bed. I enjoyed all of those things before too, but they seem more precious now that I miss her all day on weekdays. I guess its sort a absence makes the heart grow fonder situation.
It gets easier
I promise.