Having been in your position, I can say that you should keep your expectations as low as possible.

Myself, I try put out the sparks, before it turns into a bonfire.
Confess interest or make an advance before the feelings get to far gone (might require a delicate approach depending on your existing relationship), and only pursue it further if you get a positive response.
If it's a clear no go, then accept it as an unattainable goal and do whatever you need to do to move on, even if you have to create a wide buffer between your lives.

I know it's unhealthy to feel for someone and not have them reciprocate, and I feel it's wrong to break up a happy relationship.

With genders reversed, your mileage may vary.