The difference between attempting to control another and setting a boundary is whether the focus is on the other person (control) or on you (boundary).
If my partner becomes unpleasant to be around when he or she drinks, I focus on the impact that their drinking has on me and I address that in my boundary statement.
Instead of saying: I want you to stop drinking.
I say: I don’t like to be around you when you are drinking.
I might go on to say: If you decide to drink, I will not want to spend time with you while you are under the influence, so don’t come to see me until you have sobered up.
If my partner has a problem with infidelity, I can only focus on what I want and need in a relationship.
Instead of saying: I want you to stop sleeping with other people.
I say: I want to be in a monogamous relationship.
I might go on to say: If you continue sleeping with other people, I will, by definition, be unable to have a monogamous relationship with you, and therefore, I will end the relationship.
NOTE: Fundamental to the concept of boundaries is the fact that we all have choices. When you set a boundary, there will be consequences for you as well as for the other party. The other party may choose not to honor your boundaries. Think carefully about the bottom-line statement you make concerning your boundary, because you must be willing to follow through on what you say you will do.