Blindsided breakup - How to avoid losing my mind (again)?

Hi all, It almost feels silly to start this kind of thread alongside terrible stories of long term relationships breakdowns and divorces but, please, bear with me. My now ex-girlfriend, fairly coldly, turned up at my flat yesterday with my possessions, communicating that this relationship just wasn't "working for her". We had only been together four months so far, but I thought that we had created something truly good between us, to nurture and be excited about. The exact reason for the breakup? I don't know. Any "warning signs"? Not really. Just extremely minor misunderstandings, the common kind in a new relationship. We had actually spent a couple of nights at each other's places, just a few days ago. Affection, intimacy, laughter, fun. Nothing, in the big picture, made me think this was coming. I must specify that I was coming from another recent failed relationship, which really messed me up psychologically, and I was now making a lot of progress in the recovery, also thanks to my ex's affection and care. ... And now this. Well, what do you all make of it, and how could I best avoid a " relapse"? Thank you in advance for all your help.