I think Dora's expectations in this case are self-reflective. I used to date a girl who in situations like this would usually say something along the lines of "I'd break up with me." I think that's what Dora's doing....
...She's not expecting Marten to break up with her because she thinks he's like the others, she's expecting him to break up with her because she thinks it would be justified.
Snipping a bit to just focus on these two parts. I very strongly feel that Dora's still within the scope of her self-fulfilling prophesy.
If in the past she's had manipulative guys cheat on her and then convince her it's her fault they just had to do it, I'm quite certain that would stick with a person. So, if Dora's been half-expecting Marten to break up with her (probably because of Faye), then it stands to follow that she'd see him reacting the way her exes have reacted in the past - she's half trying to guide him down that path because, as far as she knows, it's the only path there is.
That's not to say that Marten won't be a help to her, because I think he will be, it's just that Dora isn't entirely able to hear what he's saying to her because she herself isn't there yet. She is still expecting the worst of the situation because that's all she's been conditioned for.
I think I see where you're coming from, and I think it's a possibility, but I don't think it's a likely one.
Sven's said that all of Dora's previous boyfriends were douchebags, but he actually specified the particular type of douchebag. He said that they were all alpha-goth types. To me, this means one of two things: Either they were trying to be the alpha-goth (in other words, the most goth they could be), or they were alpha types who were also goth. The first one is kind of cartoonish and silly and so I'm going to assume it's the second one. I'd have to imagine most of Dora's exes have been misanthropes, possibly sadists, who thrive on domination and demand to be viewed as the superior one in any relationship. In other words, when they cheat on a girl and tell her it's her fault, they don't say it's because she's a crazy bitch who drove him into the arms of another woman, they cheated on her because she's inadequate. She's not good enough in bed, she doesn't suck his dick enough, the other girl doesn't cry when he sticks it up her ass, he's still going to fuck her because he can juggle both and dammit she should be glad he still gives her the time of day for a quick fuck whenever he says so. (
Sorry if that was offensive to anyone Sorry that was so horribly offensive period, that's not how I actually feel, I'm just writing the mindset). When they get caught, they don't try to hide it, and if she can't "deal with it" then the relationship is over. On their terms.
From there, it has to be apparent to Dora that she's dealing with a different situation. Marten doesn't demean her in public or in private, he makes jokes at his own expense and he's okay when she's the one suggesting sex (or suggesting not having sex, as the case was when they first started seeing each other).
I think that her paranoia is a lot more generalized than a self-fulfilling prophecy. Since it's apparent that Marten's not the same as the other guys she's dated, she probably doesn't think that the relationship is going to happen exactly the same way. She's just been left with extreme levels of paranoia from her previous relationships that are causing her to write a new self-fulfilling prophecy with details that are actually pertinent to the current situation. She believes that Marten's going to leave her for Faye, and the jealousy she's exhibits is causing friction which could, at high enough levels, succeed in driving Marten away, and presumably into the arms of Faye if she was willing.
Lastly, I still feel that today's strip features Dora while not in the throes of her issues. The reason she feels that Marten is going to leave her is because a lot of guys would. They might regret it later, but that soon after someone does something that horrible to you (and to your best friend), there's a tendency to overreact, or to panic. He went out for a walk to calm down and try to put it all in perspective, but in the end it was only an outside perspective that actually helped him figure anything out. In an alternate universe, say one where he ran into Tai and she was baked enough to try to talk Marten into leaving Dora (more Dora for her), he might've actually ended up breaking up with her. Her issues caused her to snap, but the reason she thinks Marten's going to leave her is because it's actually a possibility.
If Dora was acting the way she would've in previous relationships (or worse actually trying to guide the relationship down the same path) she probably would've reacted with deflated sadness to the scene of Marten and Faye in their underwear rather than anger, and when Marten walked back in, she probably would've been bracing herself for a lot worse.