Why does success feel like a trap instead of an achievement?

This is more common than the success literature admits, and it is not ingratitude or dysfunction. It is what happens when the goal was always slightly ahead of you and then one day it is not.

Why the trap closes at the moment of arrival

For years the gap between where you were and where you were trying to get gave you structure. It told you what to do next, what to sacrifice, what to defer. When you close that gap, the structure goes with it. What looked like a destination turns out to have been the engine. You reach it and find a room, not a reward.

There is also the version of yourself you had to become to get there. Driven people tend to narrow. You optimize, you cut what doesn't serve the goal, you get good at not stopping. Then you stop, and everything you cut out, rest, aimlessness, genuine curiosity, relationships you deprioritized, comes back as a question you are not sure you can answer.

The identity problem underneath it

If the pursuit was your identity for long enough, success threatens that identity more than failure ever did. Failure at least keeps the story going. Success ends it, and then you have to decide who you are when you are not striving. That is not a small question. Most people in the middle of achieving something have no practice with it at all.

This is why the feeling lands as a trap. The trap is not success itself. The trap is having no self that exists independent of what you are trying to accomplish.

What actually helps

First, take the feeling seriously as information rather than evidence that something is wrong with you. It is pointing at a question you needed to ask anyway: what did you actually want, underneath the goal?

Second, resist the reflex to immediately set a bigger target. That is the chase reasserting itself as comfort. It will work for a while and then you will be back in the same room.

The more useful move is to sit with what the achievement was supposed to prove, and to whom. Most goals, traced back far enough, are answers to someone else's question about your worth. When you find that source, the trap usually starts to open.

You are not broken for feeling this. You are just at the beginning of a different problem, and that one is yours.