Hi everyone, this is my first post here, so I hope it's a decent one

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I'm in a relationship and I often worry about the other person not being happy. I easily get consumed with the thought that there must be someone else in the world that could make this person happier then I surely can. Obviously, this person makes me incredibly happy so that's reason for me to want to stay together forever, but the unselfish part of me doesn't want to be together unless she feels happy and ultimately free.
And the worst part is, the relationship is going fine. There's no horrible signs that tell me I need to worry. I just do. I want the best for this person so bad that I don't care if it's with me. It's hard for me to believe I'm good enough, because when you love someone, you only see the good in them, and thus they look perfect in your eyes.
Anyways, I know there's no quick verbal solution to be read. But I'm just curious if this is common. Do/did you experience this in your relationship? Maybe any gems of perspective or wisdom that will point me in the right direction, hehe? I'll appreciate any and all replies.
Thanks,
Magga