What is a
soulmate? If you believe that there is someone out there who has a
divine predisposition to being the "perfect partner" for you... No. The odds are ridiculously against that. There are 6 billion people in the world. If you only have one soulmate in the world, the odds of me being your soulmate is 1/6,000,000,000. It is highly likely that your soulmate might have just died in a landslide, tsunami, earthquake or flood in the past week without you ever having met him or her.
Assuming that in your lifetime, you get to know about than 3,000 people personally (which is a very generous estimate) your chance of ever meeting your soulmate is 1/2,000,000. (To put things in perspective, the chance of you winning 4D is 1/10,000) If you're Singaporean, the chance of you meeting your soulmate in Singapore is 1/1200, assuming you don't mind your soulmate possibly being of the same sex, of a different race and religion, and of any imaginable age gap.
What am I trying to say here? The idea of "there is only one person out there for me" is romantic, but unrealistic and absurd. The truth is, it is entirely possible for me and you to develop a loving relationship if we both put in the effort and the circumstances permit it. I've known my girlfriend for about 9 years now, but there was no divine element involved in us getting together. I was just a teenage boy who thought it would be cool to have a girlfriend, so I pestered her until she said yes. It was a silly, impulsive decision (as far as silly, impulsive adolescent decision-making goes), but today we are best friends, lovers and everything in between.
"That makes you soulmates, then!" some of you might be thinking. Not exactly. What we are today is a conscious decision on both our parts. There are definitely other guys out there who would be a better fit for her, and there are girls out there who would be a better fit for me. However, you don't see either of us looking around for that 'better fit'- because there will always be someone 'better'. The value of a relationship (whether one between family, friends, colleagues, partners, you name it) is not the perceived (and very subjective) value of the individuals, but the communication, shared experiences, respect and trust between them. You can't develop this overnight.
Nobody is born a great friend, father, husband or lover. You develop and grow into your role, making mistakes and learning from them along the way. Think of your best friend- was he or she your best friend the day you met him or her? You don't meet someone and think "OMG I'm in friendship!", you develop it over time. Why do we do that for love? It's probably just the hormones and social constructs overriding logical thinking. I detest the conventional concept of a soulmate because it implies that there is a shortcut to a great relationship. There isn't. Sure, some people get along better than others, but it still takes time and effort to pursue that goal. Sometimes you get lucky and things seem to just fall into place, but there will still always be friction on some plane of your mutual existence- or else it wouldn't be interesting, and you wouldn't have any value to add to each other's lives.
If an elderly couple claim to be soulmates, I would smile and be inclined to agree- not because they were made that way, but because they grew into it over time. If anybody else does it, they're really setting themselves up for disappointment- they would end up expecting too much, and will likely be complacent and give too little. It's especially sad when a couple has been together for years and lose the fire halfway because their "nothing can go wrong, we're soulmates!" assumption made them ignorant to the slow chafing and disintegration of their relationship.
Do I ever worry about "the one that got away"? No. I do, however, think about past relationships all the time. How could things have been different? Would we have been happy? Did I do something wrong? Sometime I fantasize about how I things might have been if I had gotten together with someone I might have 'clicked' better with, or even simply someone more cool or attractive. Everybody does that. Sports fans do it all the time when they fantasize about "dream teams". It's a fun exercise in fantasy- but the truth is that no dream team in reality would be able to match up against an actual solid team with an identity to bind them, a loyal fan-base to please, a point to prove, and a legacy to defend and glorify. At the end of the day, the happiest and truest fans are the ones who go back to supporting the teams that they grew up with- not imaginary ones they make up, or whatever team happens to be successful at that time.
I assert again, as I often do, that reality is far more wondrous, beautiful and fulfilling than you can possibly imagine. Scrap the idea of an imaginary soulmate that might some day sweep you of your feet and focus on giving your best in all the relationships you have in your life right now.