Well, why not both?
I know, I know, it doesn’t sound like a question with two possible answers that are not mutually exclusive, but let me rephrase it first.
The question basically means: do you believe that happiness in life stems from finding the one right person for you, or maybe from the effort in building your relationship? I think it’s both.
And both are quite self-evident.
Of course, you need to find the right person, if you want to be happy with them for the rest of your lives. No matter how much I’d work on that relationship, I wouldn’t be able to be happy with a person, with whom I don’t share basic value system, with someone very religious, with a woman, with someone who requires monogamy... Apart from some obvious deal-breakers, there can always be just differences in character that would simply make you unable to understand each other as well as you otherwise might.
On the other hand, after you have found a perfect person for yourself, it’s not like it’s just gonna be roses and rainbows forever and ever. Even if you’re a perfect match (as I, my husband, and OkCupid, think we are), you will inevitably have some worse days, some misunderstandings... You can’t escape working on your relationship.
So is there such a thing as “soulmates”? Kinda. It’s way better than that: everyone, mathematically, has to have more than one perfect match in the world (it would be scary as hell if we only had one among the 7 billion people on the entire planet!). But it’s also way worse than that: finding your soulmate is not enough to be happy.
Well, good luck!