We do things not because they are easy, but because we thought they were going to be easy. I have no idea who to attribute this to, but evidently it's a mutation of JFK's Moon speech. Whoever it was, they were pretty cynical. Consequently, things are inevitably going to be harder and take longer than you originally thought; that's perfectly fine and just be patient with yourself. It's okay to reduce the scope of things, and it's okay to not know how everything will pan out from the get go: if rules were made to be broken, then plans are made to go wrong. If there's one thing academia taught you is that projects—actually your whole damn year—will rarely go the way you thought it will.
OKRs are good and all, but like all other models, it works for some things (e.g. business targets) and not others (e.g. long term, dynamic projects). You can easily spend all of your time trying to meticulously craft OKRs to ensure you have the right scope, the right degree of ambition, but KRs and objectives can come crashing down when circumstances inevitably change. Life finds a way, so be fluid and be flexible.