Said the guy with her wasn't her boyfriend, from the tone and hesitation it was an obvious lie.
This was her decision, besides it doesn't sound like you were then, or are now, in a state of mind to be content with anything she says or does.
Nor does she owe you anything. Healthy people lie like this too, it's just part of dating life.
She had told me before she'd never get back with her ex, that he was too obsessive and emotional, that they don't mix well, and yet she's been back with him for a month.
It's her choice. Always has been. Maybe she'll figure it out one day, or she won't. It has nothing to do with you.
does she really feel any guilt about me?
Doesn't matter. Trying to base your sense of self worth on someone else's feelings is a good way get hurt by 99.9% of people you are emotionally attached to.
She quit her well paying job two months ago.
Her choice.
I don't respect cowards that would rather save face and not deal with the consequences of their behavior.
That's your choice. You can feel any way you please about anyone you want, just don't expect it to mean anything to anyone else.
She'll run away from him again just like she did before, and that cycle will repeat until she gets help.
Maybe, it's her life to lead. She's an adult, thus allowed to do this.
I hate feeling like someone has gotten the better of me,
How?
it's going to be difficult not to vent my frustration at her.
You're an adult, as such this sort of behaviour can have consequences. Might be best to avoid interacting with her until you can be sure you will behave as an adult.
I would have been fine months ago if she'd just been straightforward with me
She's refused to have anything to do with you for months and months. You were separated longer than you were together, most people would have given up long ago. I don't see how a different wording on a sentence would have made it easier for you to accept.
Maybe with time we'll be able to be friends
I'd say let it go. If not that, don't even think about being friends for a long time.
instead it was him, he gets his second chance and I don't.
This happens in relationships. It sucks, but it's just part of life, with or without mental illness clouding the picture.