Belief about money and a client

I was listening to Brooke yesterday talking about beliefs and money (in her latest podcast I think), and suddenly I realized something.

This year, I make a good living and have increased my income, thanks in part to one of my clients (I’m freelance). I work for them for 4 years now. And this year they give me a lot of projects. This client is the one who employs me the most, and who pays the best (much, much more than my other clients). But I always tell myself that if I lose this client, if tomorrow they don’t want to work with me anymore, I lose “everything”. In any case, more than half of my income.
Of course, I’m always trying to find other clients, and not to put all my eggs in the same basket, but in 5 years, it’s a fact, I’ve never found another company that make me work this way (several days, even weeks) at such an interesting rate. So in my head, since it has never happened in 5 years to find a company like this one and a job like this one apart from them, it’s “impossible”, and if I lose them, I “go back to being poor”.

How can I get past this belief, which is based on 5 years of “proof”?
Thanks to Scholars (i love you all!) I’m changing my thoughts slowly and start to understand I can do something about it, but I still really struggle with this one. My brain is like. “cm’on, that’s just maths, you can’t change that, it’s been five years, they are the only one who can do this for you, stop dreaming”.

Thank you so much for your help!!!