I noticed that your toothbrush was gone etc etc. Then keep quiet. What you have stated is an observation, not an accusation. See what she comes back with. If she is honest and tells you that she stayed out with a 'friend' then just tell her your feelings, like 'I feel disappointed and hurt that this is your decision.'
You would have told her your feelings but not come off as a angry manic. You would have stood up for yourself and your stand in this situation. You cannot MAKE her be faithful to you, all you can do is choose to tell her how you feel in an honorable, respectful way and show her that you are not the angry maniac that she thought you were.
Not being angry is equal to being Mr Nice Guy. It's just that you don't resort to anger for expressing your feelings, that's all. (I just learned this lesson myself). You can still stand up for yourself if you speak up and not use anger in emotional situations.
I know your feeling extremely bitter. You deserve to feel this way. Let me ask you. Do you still want to save this M? Because what you do from here on now will be in light of your biggest goal. Feelings of bitterness can be fixed later on, it can with hard work. But if you damage your M any further then you won't get a chance to fix anything.
Me:39 H:40 S:9 D:7 First Bomb ONS:June 07 Second Bomb OW: March 08 Separated: March 08 M:15 yrs T:18 yrs H deep into A with OW Achieved ACCEPTANCE May 30, 09