I think often, the pain of a breakup also comes from mourning a future you thought you'd have with that person. I think that's very fair, and is one explanation for why a breakup from a shorter relationship can be devastating. If you're with someone and making future plans (both in good faith at the time) and talking about a future, especially if you're at an age where you might feel (rightly or wrongly) that you're "running out of time", to suddenly lose all that AND miss the person is extremely difficult. It doesn't matter if it was only a few months. Of course, there's an argument to be made for not getting ahead of yourself, but generally we're wired to forge connection and reproduce, and it's not wrong to start to tentatively plan a future with someone, especially if they seem to be doing the same. (And of course, there are good and bad reasons to want to be with someone, but the heart wants what it wants and sometimes it's hard to step back and really examine your own motivations.)
I'm not saying this is the case with Jack and OH. Who knows what went on there. She's dreadful, and I won't give her the benefit of the doubt, based on her past behaviours as evinced on Twitter. She probably is missing the "wrong" things.
I just wanted to offer a different perspective on why (for non-Jacks) the length of a relationship doesn't have to correlate with the sadness you feel at it ending.