Ok, so i am not sure if it was mentioned, but still: i wasn't able to locate the way you can load an existing mod.

For example:

I made a mod (let it be "abc.esm"), saved it and closed Creation Kit. Then i found out that my mod has several issues and i want to fix them. I open Creation Kit again.

Question: Do i have to use "abc.esm" as load Data? And if i do, then do i still need to check "Skyrim.esm"?

If you want to edit an exisiting mod you have to check your Skyrim.esm and set the mod you want to edit as the active file.