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Danny loves Mindy but he knows he's going to fuck up their relationship eventually and can't let that happen. He knows he hurt her when he ended their relationship, but this is the best idea for them both in the long run. It hurts now, but it'd hurt worse if they continued dating and the relationship ended badly. He won't ruin them. And losing her is not an option – he needs Mindy in his life in some way. He feels selfish even though he's just trying to protect them both.
It killed him to break up with her – walking away was the hardest thing he ever had to do – but it was the right decision. He just needs to believe it. And even though he's heartbroken right now, so be it. This is way it had to go down. He'll move on eventually (hopefully). And the pain will fade soon. It's only temporary.
Mindy thinks she may definitely be in love with Danny and can't believe how much of an idiot he's being. Why had she stupidly taken the magazine's advice and let him think breaking up was their best choice? She won't let this be a permanent end to their relationship. Mindy wants to be with him. Danny has become one of her best friends, and she wants to live out some romantic comedy with him. They deserve a happy ending and she'll make him see that.
She wishes he had given their relationship a chance and hadn't panicked (that was definitely their undoing). Keeping it a secret hadn't been their best idea either – they should have told their friends. They would have been happy!
And if she has to date other guys to make Danny jealous and realize the errors of his ways, then Mindy will do it.