First, everyone involved - you, your husband, your girlfriend - gets props for handling a potentially difficult situation really well. You would not believe the train wrecks that occasionally pop up here. Y'all are well ahead of the curve.
Second, you and your GF are deep in the throes of new relationship energy (NRE). It's the exciting 'can't get enough of each other so no sleep!, 'the outside world doesn't exist!' and so on. NRE is a physical and chemical reaction. Both of you are just awash in happy bonding hormones and neurotransmitters. It typically doesn't last very long - about 6 months up to around 2 years. Now just because it's a chemical bath doesn't mean it's not real. It is and go enjoy it! One of life's pleasures!
Now the dark side of NRE. It can make people utterly idiotic. Serious cases of Da Stupid. Leave old relationships for new; neglect work and so on. Generally the rule from many people's hard earned experiences is to make no further serious decisions while in the throes of NRE if at all possible. Don't move in together, don't have children, etc.
The dark side of NRE for poly folks is that it needs to be managed carefully by all involved. NRE can lead the partner not in the happy fun chemical bath to feel ignored, abandoned. Sometimes knowing 'this is not forever' helps as does actual joy in your happiness by your partner (compersion). But it helps to have a partner even if deep in NRE stop and check in. Which is pretty much what you are doing. And you and your husband are also dealing with some major changes to your life - you're working and away from the home for the first time and you are raising 5 children together. Throw in a girlfriend and a wife giddily exploring her sexuality, that's a lot. Make sure to find time for him that is for him and not home or kid related.
Your husband has handled it well as has the girlfriend. You - and them - can have it all. It sounds like he wants you happy and is open to communication. She respects your husband and marriage. These are fine places to start.
So tl:dr - you're deep in NRE; acknowledge and know this about yourself; don't make any serious decisions about your relationships now; you are very lucky in your partners being open and honest so far; you've made major changes to your life circumstances with your husband that will have some impact on your relationship with him and with your girlfriend.
There is SO much on NRE here. Search for NRE or new relationship energy and you will find much concrete what to do and what not to do.