When I returned to work I thought I would never stop worrying or crying. After 3 days, I was fine!
My daughter is with my mother-in-law and is doing fine (the adjustment was harder for me than for her). There is an adjustment period for everyone but a 5 month old will adjust VERY quickly. So she will cry (ok, a lot) the first day but she will stop eventually. Why don't you tell your boss that you will need a bit of flexibility the first week back so that you can run home if necessary (if that is even a possibility). Just knowing that you can run home will help you feel less anxious.
Hang in there. Your mom who you trust is the next best thing to you. And think about that special bond they will be creating. All will be fine - just keep reminding yourself of that. I think that you are very lucky - you have your mom and are not putting your child into daycare - talk about anxiety!! I give a lot of credit to the moms out there who do that everyday!!
Providing financially for your child is a part of your responsibility as a parent. Don't let anyone tell you that you are not doing what is best for you and your baby because you need to go back to work!
Good luck!!