Soulmates aren't a thing. You've only dated one girl before so how do know that there isn't someone else just as compatible with you as your ex? We like to tell ourselves that soulmates are a thing but they aren't. There are a lot of people in this world so it's inevitable that there'll be a few people out there who would be compatible with you, but they aren't soulmates. They're just people who have some stuff in common with you. Not just that, but those feelings that one may hold for their "soulmate" can sometimes die over time. There is no such thing as soulmates. Relationships are work. They require that you find someone you are compatible with and from there you have to work to make the relationship and last long. People believe in soulmates and then they become shocked when they end up all alone because they spent too much time looking for their "soulmate" or obsessing over an ex. If you guys broke up then there's a good chance that your relationship wasn't working out that well in the first place, so it's healthier to just move on rather than obsessing over someone who doesn't want you romantically.
There are degrees of compatibility though, and while you might be able to make a relationship
work with many different people, that doesn't mean that there can't be someone who's optimal for you. I wouldn't use the word "soulmate" personally but I do think the idea that there's one person out there who's exceptionally compatible with you has validity.
I'm biased on this question though because once upon a time I fell in love with someone, and though we never got together and eventually parted ways, to this day, years after the fact, he's still my "standard." I've met a lot of people in the intervening time, and even considered some for relationships, but none of them have quite measured up to that standard. I could've made a relationship work with those people, but everything was more effortful with them, whereas with him, even disagreements were easy.