So sorry, OP, and what a crappy time of year to find out. You've already had lots of good insights and advice. Unfortunately lots of women have already trod this path, and learned painful lessons along the way.
Here's my 2p..
Deep breaths, know that there are people who can help you through this, on MN, family, friends, GP/counsellors, etc. Choose wisely re family and friends; we all have our own agendas and you need unconditional support, not interfering and bossing. That's why MN is good, because people can be straight and bossy, but you don't feel obliged to do things to our timescale.
Eat a little. If you're like most, appetite will have flown away, so make it little bits of what you fancy. Sleep is also difficult. Don't rely too heavily on alcohol.
Do not engage with the OW. She does not have your best interests at heart, even if she pretends otherwise.
You will be busy, so start a list:
STI check very important
Half hour free legal advice
When H is out, find, copy/photograph and keep safe (ideally at a friend's) as much financial documentation as you can lay your hands on.
You don't have to make decisions about your eventual direction right now if you don't want to, but please please protect yourself in the meantime.
Good luck, keep posting, and don't leave MN logged in on any shared devices. And delete your Internet history.