Pamela Druckerman's New York Times opinion piece, What You Learn In Your 40s, is probably the best such one I have ever read. One wise observation she makes is: "When you meet someone extremely charming, be cautious instead of dazzled. By your 40s, you’ve gotten better at spotting narcissists before they ruin your life. You know that “nice” isn’t a sufficient quality for friendship, but it’s a necessary one."
Truer words were never spoken about dating in your 40s. If you find that you are still on a quest for love in your 40s, this is one critically important thing to remember. You must be discriminating. The things that were passable in your 20s and 30s suddenly are no longer and you have to approach things with a sharper focus. If that person is not Mr./Mrs. Right–or even Mr./Mrs. Right Now–you have to gracefully exit stage left. Your gut always tells you if something is potentially problematic (from the very beginning if you are listening, which you should be), and you are better served saving everyone's time and proceeding on your own.
This edict should also be applied to personal friendships. As we mature our circle of friends tends to grow more closely knit, which is a good thing. In high school and college we have so many friends that we find ourselves in various cliques to enjoy them all, but as an adult–especially a 40-something one–suddenly you only have a few good ones that you can count on one hand. And that's okay. These relationships develop into richer, more fulfilling, valuable ones than their predecessors. Life consists of many moving parts and you find that your more adult friendships consists of those who unselfishly and without judgement take this ever-evolving journey with you.
Much like you would (and should) rid yourself of toxic romantic relationships, you should do the same for platonic ones. This frees you up to love fully and wholly in the relationships that really matter.