I've been an introvert since I was in high school.

I hope it's normal because I've become the same way over the last few years.

You’re not slipping into insanity.
I think that unless you fit the stereotype of a man in his forties....which is the package of wife and kids than people can’t step out of the box to engage. I think it then becomes simply issues of discussion and for some reason people are just really afraid to go beyond very surface level discussion.....I guess it makes them uncomfortable.
I’m the opposite....I have trouble simply doing surface level discussion.

I just find it hard to engage with people who don't/won't bother to engage with reality and live their one life but instead waste time on distractions and delusions.

Im becoming more solitary but when Im around people, Im still as talkative as I ever was

just slipping. get out there and join something. anything will do .

Maybe you're tired of people and more impatient?

I have also found that the older I get the smaller my circle had become. It’s almost nonexistent at this point. Much of this has been by choice. My own personal/spiritual evolution has caused me to outgrow most of my personal relationships. I’ve found that over the past three years, my social life is lacking. I’m now putting myself out there more. Trying to meet new people who share my newly developing beliefs and interests.

I think it'sw happening to me as well. In the sense of needing more "recharge" time after being around a lot of people. Wehn I was younger groups of people energized me, and now they wear me out! Also, I work with the public, so it can be constant.

...the insanity one....

I can't help, but I may be making the opposite progression. I'll wave if I see you.

The older you get, the more you realize all the ways things can go wrong if you interact with anyone. It makes you a bit more introspective and cautious before you engage with people. That's not being an introvert, and it certainly isn't insane.

Natural for you perhaps

I think that experience has made you more selective. You don't waste time on relationships that do not meet your qualifications.

We all go through changes. First I got my toxic friend out of my life-emotionally needy, current/former addicts and dysfunctional family. You need to take care of yourself and do what's best for you. Maybe you needed time for yourself.

Me too. I like people less and less as I age; I have less tolerance for "fluff". So I don't know if it's introversion per se, but I dislike crowds. Being agoraphobic doesn't help...

why would you jump to thinking of insanity?

No you are not slipping into insanity, you are just being more selective in the company you keep.