at the beginning or in the beginning?

Well, they are both correct.

When I hear or read "in the beginning," I always think of the Bible. "In the beginning, God created..."



So here's the trick. In the beginning is used on its own. But at the beginning needs to be linked to a prepositional phrase (whether written, spoken, or imagined.)

For example:

In the beginning, I found the project difficult.
At the beginning of the project, I had a difficult time adjusting.

In the end, the project turned out to be a success!
At the end of the project, we had to review and analyze the outcomes.

You will find that "in the beginning/in the end" are often more general statements.