A couple of suggestions. First off, I would take what your H's coworker says with a grain of salt. He's not you and he's not your H. He really doesn't know what's going on and your H could very well be just enjoying the company of a friend. Just because he doesn't show that he's miserable like you may be, doesn't mean that he isn't.
"she laughs at his stupid jokes."
If she does and you don't and you think of them as "stupid" is there any reason why he may not feel comfortable around you?
It doesn't sound like he was lying to you just because he's friendly with this girl.
You mentioned you had mood swings. Maybe you need to take a deep breath whenever you hear something like this so you can think clearly rather than going on impulse. Will it help or hurt your situation?
Not saying that you're not right, but sometimes it's better to think things out a bit and see what the concrete facts are and not circumstantial evidence.
M-43 W-40 2D - 9 and 5
Emotion, yet peace. Ignorance, yet knowledge. Passion, yet serenity. Chaos, yet harmony. Death, yet a new life.
