Pretty lonely to be honest. Can't stop thinking about what I haven't got which I know isn't good
Don't fret. We all do - and especially at times like this when it's supposed to be some kind of milestone. It makes people reflect upon their lives, and that's normal.
FWIW, I grieve for my dad (who's alive and kicking, by the way) because going home for Christmas has, once more, shown me just how little he cares about anyone but himself. I'm sad that, see my 1st post on this thread, I've sort of lost my best friend (even though he's still my friend; it's just not the same when you can't ring them and pop over for a drink and bitching session). I wonder how my ex and the family that was once meant to be but ended up not being mine are doing.
But that's alright, isn't it? Because it means we're capable of caring about others. And, really, there will be people to whom you mean the world, and they need you to have that capacity.
It's really, really going to be fine! 
Have a wonderful 2019. You'll do fine, I'm sure.