Welcome to our forum.
With the amount of stuff you're juggling (e.g. five kids and a newly-started job), you'll have to schedule your time really carefully. It sounds like your husband is in need of a little more of your time and attention. See if you can work something out along those lines with your girlfriend.
Hopefully you are okay with it if your girlfriend finds a partner as well. Even if she doesn't, though, people don't always need 100% of their partner's time. I am in an MFM "V" where there is one gal and two guys. The guys just have a platonic friendship, so we are sharing the lady's time and energy. You might suppose that both of us guys feel shorted, but actually I think we are both pretty okay with the arrangement. Everyone is different, and every relationship is different.
Re: coming out ... if only polyamory were widely accepted, we wouldn't have to live in the closet like we do. Some people announce themselves to the world; others have to keep it a secret. Alas, I am in the "secret" group. You just do the best you can with your situation. Polyamory is not for the faint of heart.
Keep looking around on our site, and post any thoughts or questions as they come to you. I would say keep your girlfriend for now, but schedule a little more time for you and your husband. It's not a perfect world, but don't give up your dreams too easily. Give poly a chance; learn as much about it as you can.
I hope we can help. I think we can.
Glad to have you aboard.
Love means never having to say, "Put down that meat cleaver!"