Breathe!!!
Then work out what you can change and what you can't. Write a list, draw a picture, or do whatever you need to do to get the facts down and out of your brain. Put a line through or brackets around the ones you can't do anything about right now (or ever).
Then put numbers by the rest - a) 1-5 on how important it is (life or death type = 5, who cares = 1) and b) how soon it has to be done (NOW NOW NOW = 5 or in a couple of months... = 1). Add the two numbers together and you can the prioritise which ones to deal with first.
Once you've done that and probably thrown it away because it tells you already what you know, stay calm!!!
I am guilty of stressing as the next bird... Teacher in a difficult school, nearly 2yo DD, just got a BFP for #2, selling house and DH trying to change jobs. I strop around the house moaning for England about it all and how life isn't fair, BUT I cope. Just. I just have to give myself a break and work out what I need to do and what I want to do, and where the 2 cross, I can be happy. Most of the time!!!
Are you going with DH to the new job immediately? Can you rent your house out? Can you wait to sell it and meet him there then?
Are you considering a termination? If you genuinely don't think you couldn't cope with a second DC, then it may be the way for you... However, most women panick (including me!!) about how to manage two and yet when they have #2, they manage. Then have #3, #4... Plus the age gap should make it easier re; schedules and routine to some extent.
Anyway, I can't solve anything for you and hope others have some good suggestions, but big virtual hugs to you - I can empathise with some of your stress (if not all) and hope you can work things out. :)