Okay, all this is just my gut reaction to your story but I have no other basis for saying any of it.
First, I think that it's likely that if this caused a breakup, then the breakup was inevitable. Just something to think about.
Second, my thought on reading that the fortune-teller predicted your ex would end up with someone with the initials "SM" was that it seemed like a
rather limited reading of whatever his or her source was. I'm not a fortune-teller, nor have I ever had a "professional" reading done (though I
have had friends who did professional tarot readings do free readings for me, and I read the tarot myself). In my limited experience, though, signs
usually have some wiggle room.
I don't mean that in the sense of "it's all so vague, you can read anything into it" but in the sense that I might for instance see a person in a
partnership role in your life, without knowing right away if it's a marital partner, business partner, etc. So I would wonder if the initials were
necessarily literally the person's initials, or if they could be initials of a nick name, career, location, anything associated with the future
mate.
Also, I think that we have free will and control over our future, at least in the details if not in the patterns.
If you really think that your insecurity based on this reading was the main cause of your breakup, I would strongly urge you to do what you can to
repair the relationship, and to work on trusting that things can work out if you will let them.
If this reading convinced your ex that the relationship was doomed as well, there's probably not much you can do.
Whatever happens, remember that even though in my opinion there is validity in fortune-telling, not all fortune tellers are valid. Maybe you just
happened on one who's a fraud