Here's the advice I would give to myself if I were a first year academic...
You don't know everything - and hopefully never will
I remember my first class vividly and one of the most striking things during the session was the seeming expectation for me to have all the answers. But even worse, I was expecting myself to have all the answers too - why, when did this new me appear? The reality is no one does. You've done your prep (poor planning leads to P.P.P and all that) yet there will always be a curve ball question. Do you know what? That's fine, just say you don't know. It's not like the walls of the University are going to crumble when you utter those words. Find out the answer (encourage your students to find out too) and come back to class equipped with a little more knowledge. We shouldn't have the perception that somehow a PhD or some other qualification gives anyone a pass to stop learning.
Perspective
There are lots of aspects to being an academic some days will be the most rewarding, exciting and brilliant days of your career. Like graduation day - it never loses its polish for me. It's the best feeling to see your students graduate and I always take a moment to enjoy it. Meanwhile, other days may present you with a whole host of challenges: a growing inbox, material to prep, an endless to do list, meetings, teaching, supporting your students, admin and some project that you signed up to but totally forgot about. Sometimes you just need to step away from the inbox, go for a walk or have coffee with a colleague. Those 15 minutes will help clear your head. You will feel ready to tackle the tasks that lie ahead and will avoid sending a s***y email in the heat of the moment. If it all gets too much just keep things in perspective - if its not going to matter to you in five years then don't sweat it. My institution is located across the road from a hospital if that's not a good enough reminder then I don't know what is.
Ask for help
You want to do a good job, you want to be a good colleague and a good lecturer. I get it. But unless you are teaching in an broom cupboard somewhere in the middle of nowhere go and speak to your colleagues when you are struggling with a new situation or question. There is experience lurking in every corridor so pop your head around the door and ask. I've never heard of a colleague (or me for that matter) be given a hard time for asking too many questions.
10,000 hours
You will not master how to be an academic in the first year. I am a firm believer that regardless of the profession (teaching or otherwise) you do need to rack up those 10,000 hours of experience. Not to say that you won't be amazing, but by having that experience you will be able to make decisions faster; judge a situation better, know when to push and when not to. It becomes a little more intuitive and you start to find your own style in how you do things. So just be patient.
Scrap perfectionism
Despite your natural tendency towards perfectionism (oh yes, I'm talking to you) it isn’t a wise strategy to be a perfectionist at everything. You need to decide which things are worth your time, attention and effort and those which are not. For some tasks they are not worth maximum brain power, sitting up to the wee hours tweaking. Get it done. Save your energy for the tasks that need it most. If the idea sends you crawling up the walls, just try it once and see.
I have more to say on this but I think that will do for now.