Headcanon that Harry becomes an auror straight out of school and, at first, Kingsley is very pleased with his work. He always does what’s asked of him and he works very hard… too hard.
The guy’s eighteen years old and has been fighting a war against a genocidal maniac since he was eleven. Now they’re sending him after petty theives and people breaking the statute of secrecy and he can’t seem to grasp that it isn’t the same.
He’s used to the idea that, if he doesn’t fight, then the people he loves will be murdered in front of him. So he never takes a break and often works long shifts into the night. Kingsley gets used to the owls that come from Ginny every few days saying that Harry hasn’t come home again. Sometimes he’ll find him working, but more and more as time goes on he’ll just be slumped over his desk, his pen still in his hand.
Ron calls him an idiot, Hermione tries to convince him that he no longer has to treat every case like he’s fighting Lord Voldemort and Ginny jokingly says that she might try using a body bind curse on him, just to get him to sleep. But no matter how much he tries to relax, he can’t let go of the feeling that everyone’s lives depend on him.
Then one day he just collapses and they can’t get him to wake up. He comes round a few hours later in St Mungos with his friends all standing over him (including Professor Mcgonnagal, who came over on the floo network the moment she got Ginny’s owl). The first thing Harry does when he comes back is try and get out of the bed and back to the auror office— he was in the middle of some important paperwork, and what if they need him to go out into the field and he’s not there? He gets so agitated that the healers end up having to give him a sleeping potion to keep him from fighting off his friends and running back to the Ministry in his pyjamas.
After that they all take turns sitting by his bed, talking to him or playing gobstones or exploding snap (not wizard chess, though. The last thing they need is to remind him of the old days).
Eventually, but only after the healers have given permission, he’s allowed to go home— though Ginny refuses to let him go back to the ministry for another month. When he returns, everyone acts very pleased to see him (they all visited him while he was ill and sent him mountains of cards and chocolate frogs) and he’s called into Kingsley’s office.
There Kingsley tells him that he’s not going to be put back in charge of case work just yet, nobody (except Harry of course, who insists he’s fine) but asks if he wouldn’t like to try training the new aurors?
Harry can’t see anything to do but agree, so he starts work and discovers that he’s really good at it. It’s a lot like being in the DA again, actually, only without having to worry about Umbridge or the Inquisatrial Squad. To his surprise, Harry finds himself actually having fun at work, rather than worrying all the time.
In a few years he even get to train Ron, who jokingly calls him ‘Professor Harry’ and later, seriously asks him if he’s okay about this, since being an auror was always his dream and now Ron’s going to be doing it instead. Amazingly though, Harry realises that he doesn’t mind at all and is quite happy to let Ron go ahead and spend time in the limelight, while he works behind the scenes.
He ends up working there for several years and, at the end of his career, all his surviving students (which was a lot more of them than there could have been, thanks to Harry’s teaching) gather in the ministry to thank him. In his goodbye speech he calls his time at the ministry “the happiest time of my life.”