His business is everything to him. He said it was the same as me basically cheating on him, it was that big of a betrayal.
Bullshit. It isn’t cheating and if he says so he is gaslighting you.
He has trust issues and I handle the finances and access to the business bank account and he doesn't think he will be able to find anyone else he trusts to do that, so if I don't do it then it will mean he has to reduce the business down to just him.
That’s not your problem to solve. That’s his way of keeping you tied to him forever.
I have been feeling really conflicted over everything because things have been going well between us
Things are not going well between you. He has just told you you’re not allowed to take another job.
I’ve been having some counselling which is helping me and I'm feeling a lot happier and calmer
What has he been doing to address the problems in your relationship? One person taking counselling to make them happy with a relationship is not how it works.
I think he will ask me to see proof.
Another reason to leave. My husband would never ask me for proof of something I’ve said I’ve done.
I described it to my counsellor as my realistic dream job
Well, unless you have another dream which is to be tied to someone who doesn’t have your best interests at heart, and to never have what you want, then sure, reject the job.
You were afraid to tell the one person who is supposed to love you the most that you got your dream job. If that doesn’t give you cause to leave him, then nothing will.