If you find yourself plagued by doubts, and an uneasy feeling of being adrift, rest assured: you aren't alone, you're probably too smart for your own good, and it will get better eventually.
Just let yourself drift sometimes, and try and forget things: the most creative answers come to us in the shower, not at our desks. The most exciting breakthroughs get triggered by totally unrelated nonsense, not reading the brief again.
And often, being happy is something we only realise after the fact: we're sad, and we look back on 'happy' times, but at the time, they were just 'times' (this is an appalling sentence, but you get me).
Kind of like how you don't appreciate how easy swallowing is until you get a sore throat, and it KILLS to eat or cough, and you think 'Damn, next time I'm better, I'll be really happy about it." You probably won't. Swallow right now. Breathe in through your non-blocked nose. See?? It's all ok.
Above all, just give yourself a break. Try and step out of the spiral. Remember happy times, and relax.
Leave it all up in the air, and listen for the laugh through the wall.