I am not qualified to answer your question, but if I may ask:  Why do you want to settle down to just one?

I mean, isn't more the merrier?

I suppose one answer I can give you is that when the right lover comes along who makes you uninterested in seeing anyone else, that will solve the problem by itself.

I doubt that there is any pill or practice that would do the trick, but I could be wrong.

I'm not a "social scientist" like LG with her various degrees, but I think she would not object to me answering your question in the following manner.....there is no simple answer to your question.

Different people have different needs and expectations. Some of us are wired for monogamy or a limited number of sex partners over our lifetimes while others require the exact opposite.
There are many factors which play into it: genetic predispositions, psychology (including personality disorders), religious upbringing and the expectations within our socio-economic class and regional settings are just a few examples I can think of.

Another factor is how our needs change through life stages. Some of us begin to settle down as we get older while others who lived a fairly conservative lifestyle in our youth, suddenly "want it all" before we feel it is "too late."

I miss LG. I think she would provide some informed answers on a topic like this one.

I wish she were still here. She could be testy when she encountered opinionated ignorance and even at times when there was a simple misunderstanding but she provided valuable information hobbyists and providers can't get anywhere else.

I have met some women (non-pro's) whose husbands or boyfriends liked them to go out and fuck other guys then come back and tell them about it.  Most guys are so possessive that I think a lot of women don't realize that such guys do exist.

I'm with PG to a large extent.   We live in a society that holds monogamy as its ideal, and makes it very difficult to discuss alternatives.   Some people feel called to monogamy and buy into the model, and even regard those who are polyamorous as having some kind of mental or personality issue.

The question to you is - why do you want to settle down?  Is it a calling in you, do you want to have kids, do you feel somehow it's the only "right" thing to do?  Understand why and it will help you figure out "if", "when", and "how".

Myself, I bought into all of it and committed myself to marriage at a young age, when I did not even know myself, my wife didn't know herself, and I thought I basically had to compromise my own self (not just actions) to "make my partner happy".   I spent the next 15 years learning the only person I could make happy was me.  Other would have to make their own choice to like, love, or be happy with me.  And sadly, if I was to be happy there was no alternative to being who I am.  I believe that applies fairly universally.

I've elected to remain single, live single, and have multiple relationships that do not demand anyone else give it all to me.  It's a lonely place at times, and its very difficult having multiple intimate relationships openly, which is the only way to avoid terrible heartbreak.    But that is another story.

I'm not going to blow sunshine up any orifice and say something like...  you'll know when you meet "the right guy".   If living the way you are is getting old or making you feel bad, then its time to change.  Get to know how you need to live and then figure out how others get to play.

And remember advice is always worth exactly what you pay for it.