If you don't want a space between lines when you hit "enter", hit "shift" & "enter" at the same time. If that doesn't help, I don't think I understand the problem.
EDIT: I thought of two ideas that might help.
1. If you're using the Simple Post Editor, try the Advanced Post Editor. (That's what I use exclusively.) Check out this thread to learn more about the two different Editors:
https://budgetlightforum.com/t/-/9068
2. When you're finished writing a post, click on "Preview" instead of "Save". That way you'll have a chance to see what your post will look like before you publish it.