Don’t sweat it. But. Find another.
First, of course you are at an extreme concentration risk. No VC, or at least very few, would want to invest, for example. It’s way too risky if you lose this whale.
What I’ve learned is that 9/10, where there is one … there are two. And then if you can get two … you often can get 10.
So break it down, and simplify.
First, of course, keep this huge customer happy.
Second, make sure you’ve mapped out every potential customer that is remotely similar. Find the key stakeholders there. And reach out with your amazing case study of how their competitor / peer is using you!
This will work.
At least, most of time.
It just takes time.