Final Cut Pro Library 10x larger than original files

Hi everyone. I'm just getting started with FCPX and already struggling with library organization. I have all my media (300 GB) on an external hard drive (2 TB with 700 GB used for other things). My FCPX Library is on the same hard drive.

I'm creating a video which is about 40 minutes long so far and I'm through maybe half my original media, but the library is already larger than 1 TB and I can't continue to edit/add clips. Apparently I missed to select that not all the media needs to be imported into the library (I think I don't need that since they're both on the same hard drive? Is there a way to change it for already used clips?). But I did the change now, but still after adding one clip it says it shuts the library down to safe the project, because there's no space left.

I read about deleting Generated Library Files to save space. Can I do it if I'm still working on this video? Since I'm just starting to learn everything about editing I'm really slow and definitely won't have the motivation to start over with the project I already spent at least 30 hours in. Or do you have another idea on how to free up space so I can continue to work?

Also any other advice on how to organize my files and library better is highly appreciated! Please excuse any mistakes, my English is quite poor I guess. Thank you!