This is really driving me mad.

I have been working for about 3 hrs now and everything was fine. I tested one piece of code, it worked and I inserted it into its correct place in the overall construct. Then I tried to step through it line by line - but that didn't work. Always after opening another worksheet, the VBE suddenly stops the debugging_mode and executes the remainder of the script in fast mode.
That is annoying because when something is not right, it is near impossible to see where exactly something goes wrong, it's too fast.