Ramona.....If by "how to handle," you are asking how we individually "cope" with people who strongly believe their own lies & illusions.....I feel comfortable offering my answer.
The reason I felt the need to qualify verbiage was for my sake because I am terribly literal and exacting (sorry) Frankly I wouldn't even attempt to "handle" someone with a mind set in concrete and closed off to any & all suggestions to the contrary. Fortunately it's no longer my job to try to provide the information and opportunity for someone to help themselves and/or improve their life. I've been retired many years.
However, because we all find that we must encounter individuals like this, perhaps live or work with them, it's fairly important to learn to cope. Yes, it can be frustrating, but it is necessary nonetheless for the sake of harmony.
I see Mackenzie has a healthy attitude. "Live and let live," is one convenient approach and works just fine for many. Keeping this attitude is great for avoiding the stress these delusional people can cause.
Ramona, I realize it can be beyond stressful, perhaps even heartbreaking when the someone who is "going nowhere" because of their flawed thinking is a person we happen to love. When we find that someone who means a lot to us has paralyzed their own progress through faulty thinking, of course we want to help.
In these cases, the best thing we can do is practice tolerance and patience while offering some guidance and direction at the precise moment, in the most gentle ways possible.
There's no way around the fact that this is not easy but it can be worth it at the end of the day. The most profound thing we can do is show the positive results in our own life as proof that there are better ways to think and believe. Good luck. Peace, Paula