If there's one thing I know, there's not such thing as the myth of "the one", on a lifetime you could have multiple soulmates.
The thing about relationships is that people change over time, the person you are today is not going to be the same, and same thing with your partner. When you break up with someone is because things weren't working out in the relationship, but then when you look back on it some time later we tend to see things through a rose colored glasses, and we think that things were better.
The fact is if you did go back with your ex, in 6 months or less you would break up again because the same things that bothered you before are still there, and trust me, they will bother you again.