Diversification of clients certainly improves your financial security, and it should reflect on your psychological state. But more efficient tool, I think, is growing your bank account to have at least 3-6x of monthly expenses. In that case you'd have more than enough time to adjust your work life in any worst case scenario.
Here's the thing: there is no certainty. Job security is sadly largely an illusion today. People who feel safe from job security just aren't realists, for the most part. I prefer being a freelancer and facing that uncertainty over putting my head in the sand until the flood comes and I have to learn to swim in 3 seconds.
Creating a buffer and investing in client relationships is the only way. Just doing your job won't cut it. You need to be known too and that takes time. How you do it depends on who you are. Extroverted people might like speaking or podcasting. Introverts might prefer writing. Either way, you cannot afford to be fungible.