Sorry, I just want to add something. It's a method I use when I know I am going to freak out about something.
As an example, I can be quite scared of flying, as once I was on a plane and the engine caught on fire and we had to make an emergency landing. But thanks to various life circumstances, I have to fly a lot. So, I tell myself that I can only be scared in "levels."
For instance, I say to myself there is no point being scared until the flight attendants are asked to fasten their seatbelts, because if they're still walking around, there's obviously nothing to be worried about. IF it gets to that stage - and it rarely does - I tell myself that I'm not allowed to be scared until the oxygen masks fall down. Which has never happened, btw.
It is just a way for you to allow yourself to have that moment, but ONLY IF it's necessary.
To relate that to your H, I would do this: Say to yourself I am only allowed to be upset/freak out/be sad if I find out that he is seeing someone else. In other words, your reaction is only allowed to come from an actual action, not a "what if" scenario.
