How can we cope with difficult people in our lives and not let their behavior affect our state of mind?
I understand how you feel. Here is the best strategy in my view. Rather than focusing on the problems, which seem to have a life of their own, focus on your self-realization as you are already doing. For example, sometimes a person lives in a house that is so troublesome, so many things to fix, so broken, that the owner can spend his or her entire life trying to fix it and even so it will never be a good house.
Solution: build a new house. By that I mean, accept the dark side for what it is, and focus your energies more and more on building a spiritual life. Ultimately you are the soul, not the depressed mind. The soul has to rise above the mind and look down upon it and observe it’s problems. But the soul lives in its own spiritual space. So you have a real opportunity to build a powerful spiritual life.
Another example, when you are attached to an unkind person, every word they say can hurt you and you can never be truly peaceful. But when you overcome that attachment, that person loses the power to hurt you.
So, the mind is what it is — a material element that can bring us great suffering. But by strengthening your relationship with Krishna, nothing the mind says or does can bring you down.