Final Cut Pro has a special way that it catalogues your progress on a project and these project files should be addressed specifically so that they do not get lost in the shuffle.
Keeping a Catalogue
Project organization is key to all non-linear editing systems because they are essentially a media management system that arranges and compiles data files. Without keeping your progress organized you can easily lose work that you have already done. This is important to use within your work file, but it is also important to keep those files itemized themselves. Final Cut Pro is a somewhat unique software in that it unites professional tools with amateur learning curves and there are specific ways to keep your progress correctly stored.
The first thing you have to remember is that Final Cut Pro remembers things that you have done. This means both what you have last worked on and where everything is. That is the main reason why it is important to keep all media in specified folders to avoid disconnected media, but it is also the reason why you should be concerned over separate projects. Final Cut Pro will open up the last project file you had saved if you try to open it blankly. This can end up causing you a number of problems so try to open directly from the saved project file instead of just opening up Final Cut Pro and then trying to open your project file in the software.
When you save, especially autosave, your saved file is then added to the last file created. This is actually a very complicated process that ends up creating a huge number of project files in the Autosave Vault. When you go here you can look at all the saves done to your base project file on different dates, though often times the dates of work are not correctly indicated by the dates of modification on those files. This can make it next to impossible at times to find the right file that represents your most recent amount of work on your editing project. The best way around this is to Save As a new project file every time you are working on your film. Just save a new file with an appropriate title, perhaps something indicating the date when you worked on it, and then next time you come to work on your project you can open that one knowing that it is the most recent save.
These are the ways that you can avoid disheveled work chaos where no file seems like the updated one. If you accidentally open an old work file make sure not to save it and to close it immediately otherwise you could end up with problems. Final Cut Pro could then think this is the most recent file and automatically open that up when you open up the software. Instead always go to your specially labeled project file to open Final Cut Pro.