As a marriage counselor I see couples sit on my couch every day who lament because they think they married the wrong one.
They tell me they should have thought it through more before they got married, because if they would have chosen better then they wouldn't be in the mess that they are in in right now. And now they're trying to salvage whatever they can - if they can. They worry that they might have picked the wrong one and perhaps there was someone else out there who they would have been better off with.
Honestly, I can't blame them for thinking that, either.
In the U.S., this idea of finding "the one" permeates our culture. It comes from blockbusters that proclaim such famous lines as, "you complete me" to songs that state, "I knew I loved you before I met you." It's a popular notion that there is one person out there that fits you so perfectly that it's almost as if you were meant to be. It makes sense, then, that when clients sit on my couch with problems they're wondering if they picked the wrong one. Afterall, if they picked the right one, they wouldn't be having these problems.
No one person can complete you - not even your spouse
The truth is, as a human you are multifaceted and infinitely complex. All the needs, wants and desires that you have cannot be fulfilled by one person. You need diversity. You need friends, family, colleagues, acquaintances, etc. You need hobbies, interests, alone time, down time, up time, etc.
To expect one person to fulfill all the needs you have as a human is going to create a lot of frustration - for you and your spouse. It also puts a lot of pressure on your spouse to try to be "the one", because this means they have to anticipate what you need, when you need it and do what they can to meet those needs.
While it might sound really great to find someone who fullfills all your needs (it maybe even sounds romantic) the truth is, this makes your partner a chameleon and servant who is supposed to meet all your needs instead of being a partner. It doesn't give them much freedom to be their own person, either, since they're supposed to be completing you instead of nurturing their own interests, hobbies and friendships.
It's more rewarding and fulfilling to complete yourself
Because another human cannot complete you (not even your spouse), the better option is to complete yourself, instead. Nobody knows you as well as you do. And instead of expecting your partner to read your mind and anticipate what you need, you can do it for yourself.
There's nothing more rewarding than realizing you are capable of being everything you need to be happy in life and realizing what an incredible person you are without needing others to tell you - including your spouse. You can get to the point where you no longer need your spouse, you want your spouse, and you want them to participate in your life because you love them and like their company - not because you can't do without them. This takes pressure off your spouse to mold themselves to be who you want them to be in order to complete you and you develop a more mature relationship where you love your spouse for who they are instead of how they make you feel (which is ultimately selfish).
So, instead of expecting your spouse to be the one for you, focus more on being the one you want to be. Instead of needing someone by your side to complete you, you'll be inviting your partner along in your journey as you discover yourself. They'll join you because they love you - not because you need them to complete you.
That's when you'll see your love flourish. And when they join you because they love you (and not because they feel compelled to), you'll know that they're the one for you.