I've been in your exact situation, trusted her all the way with the 'just friends' thing, she ended up cheating on me with the guy (and I had major reasons besides my pride and my trust to think she would never do that, too), she tried apologizing and doing her best to piece everything back together between us again for a few weeks because she could tell I was wrecked and then she just cheated on me with that guy again.
She had my entire trust the whole time so I thought it was 'nice' and cool letting her see her 'friend' till one day the news broke, and then broke again. So in my experience that's a very slippery slope, if you ever feel like discussing your situation because it definitely sounds familiar to mine and I know times like these really are confusing, don't hesitate to PM me.
Your last paragraph also spoke to me because the whole time she was doing this to me, she was really making me feel like a sack of shit blaming me for all the problems in our relationship, basically saying I was busy working too much to get to spend time together even though we actually spent a lot and I was making conscious efforts to organize myself even better. Now in hindsight I realize that in reality, she was probably just looking for excuses for her to get away from the relationship and feel better about living another romance, and she never had our common interest in mind in the first place. I mean of course if you're going through that shit in addition to your depression, her seeing that guy is never going to solve any of you guys' problems and maybe you should make sure she realizes that. My ex didn't even notice she was drifting away from our relationship and personally harming me because she was so passionate about that other dude (something "light-hearted" as you said) everything else stopped to matter, and she genuinely thought she wasn't doing anything wrong till it hit her that she fucked up and had lost me eventually.
Sometimes people get those urges and then for a while they don't know what they're doing. I'm not saying you shouldn't trust your girl personally for not having virtue (being outspoken about your feelings like you guys seem to be is great virtue), just that you shouldn't trust her more than you could possibly trust another person who's also a human being with their weaknesses and limitations. Be realistic and like you were saying, focus on yourself because in crucial times like this (which I wish won't be so crucial to you) one really needs that.
dear BeG, flip the script and hang out w/ a girl you like telling her 'trust me, i can not make out /w her sweetie' and see if she goes for it. if she's ok w/ it she's planning to let this guy pet the pussy. if she says no, she still might be ready to let him pet the pussy but she's not willing to extend you the same courtesy. in this case you have what is called a 'hotwife' not an open relationship.
DTMFA! [dumpt the motherfucker already]
unless that's your kink
Funnily enough my ex was really encouraging me to get a side chick the whole time she was seeing that 'friend' of hers she was in the process of developing feelings for; what terminated that stress was the event of me actually starting to talk to another girl (after I had been cheated on and made it clear our relationship was now over). Now she's freaked out about the idea of me getting involved in a new relationship, and keeps saying (all the while still seeing her new dude) she wants me back. Fuck that man, I'm running for my life here.