I am sorry you are hurting.
Could start from the first floor on up. Do your self care and take care of basics first -- get sleep, eats, take care of the kid.
Could work your way through Maslow needs
one chunk at a time. You don't have to do them all in one go.
While I don't agree "sex" is a basic first floor thing, could still start with the other first floor basics and then move up to safety things. When you get to thinking that level out -- secure the joint finances into "his and hers" so you aren't left up shit creek in the transition time because she withdraws it all. She seems to be making fast/rash decisions.
Make mediation/divorce lawyer appointments, etc. Try to be as reasonable/sane as possible -- your kid needs you to.
Look after your wellness dimensions --
BODY: If you need a check up, need help sleeping, see your doc.
MIND: Try to make time for destressing things like your outdoor fun.
HEART: feel whatever it is you have to feel. Let it blow on through even if some feelings stink to be feeling.
SOUL : I'm not sure what you do for spiritual health, but do it. Maybe it's the nature walk thing, or you have a minister to talk to or journaling or art or music... try to make the time to tend to your spirit. You seem in poor spirit right now.
Your wife wants to divorce.
Your GF is supportive, but concerned what that might mean for the future.
You are worried about the future and forming new polyships.
All understandable... but how about worrying about things in time order, one hour at a time? One day at a time. One week at a time. Through that lens... preparing for divorce comes next. Wife already moved out -- figure out your divorce laws in your area.
Worry about your status with the GF and what your next polydating life will be like when you actually arrive at that stage. You might have the capcacity for polyshipping, maybe even the desire for polyshipping... but is this the TIME to start new polyshipping? Nope. It is not the time for that stage.
There's other stages to get through first that require your attention right now.
Hang in there.