The brain is legitimately wired this way. It's called hedonic adaptation - we get a dopamine hit from *pursuing* something way more than from actually *having* it. That three-year climb gave your brain consistent stimulation, progress markers, something to solve. Once you hit the goal, there's no more puzzle to chase, so your brain feels... flat. That's not a failure on your part, that's just how the reward system works. Same reason finishing a book or game can feel anticlimactic even if you loved the experience - the journey was the dopamine source, not the destination.
The tricky part is that most of us aren't taught to expect this, so we assume something's wrong with us. We think "okay, big achievement unlocked, now I should feel great forever," and then feel guilty when we don't. But the fix isn't trying to force happiness about the promotion itself - it's understanding that you probably need the *process* more than the outcome. Some people build their lives around that by always having a few things in motion at once, so they're not relying on one finish line to feel fulfilled.
With hobbies it's a bit different though. If gaming isn't scratching the itch anymore, maybe try starting something new rather than finishing things? Or mix up what you play - the hollow feeling after beating something is real, but jumping into a fresh challenge usually resets that. The promotion hollow feeling will probably fade a bit once you're settling into the new role and finding new problems to solve in it. Give it time before you decide you need another promotion to feel alive.