The mind is its own place, and in itself
Can make a Heav’n of Hell, a Hell of Heav’n
A quote from John Milton’s Paradise Lost.
Interestingly, the idea that it’s not solely events themselves that determine our emotional reaction but also how the mind makes sense of them, is a core principal of CBT or cognitive behaviour therapy.
Mind Hacks 
I dont think the quote has anything to offer to modern thinking. What it says has been reiterated time and again and so the thought has lost its uniqueness.
You are so blessed to have a mind so stable that it does not require protection, especially when we find that it is the mind that governs everything about us. Interestingly, when the body was created during days of antiquity, nothing has changed today, including the design of the mind.
I think the quote is timeless. I have a sister who is mentally ill and makes a life science out of the latter part of the statement. My take on this quote is that we are capable of deriving happiness in the most terrible of situations and, unfortunately, making pleasant situations utterly miserable.
I think this is one of the most insightful things ever said, and its force is unbroken. The mind can blunt the edge of the worst the world can throw up, just as it can ruin any good thing we have going for us. Solzhenitsin well put essentially the same point. We have an unrivalled ability to get ourselves into scrapes of our own making, just when all seems plain sailing and nothing can go wrong; but we also have a mysterious ability to get ourselves out of the most disastrous situations, when all seems hopeless and all seems lost. Somewhere between the two extremes lies the human condition. But Milton put it best. It’s a bumpy ride being human. Welcome aboard!