Since you tried without success the things I mentioned in my earlier post plus other things, possibly the issue is related to the recent update to iMovie 10.1.15. (Are you running iMovie 10.1.15?) There have been reports of the 1080p sharing option to You Tube being inoperable with iMovie 10.1.15. However, sharing in 1080p to the desktop has been successful. Is that possibly your issue? You didn't mention where you were trying to share your projects.
Another thing is that iMovie supports sharing 1080p at 60 fps, but may not support 1080p at a higher frame rate. Also, to cover the bases have you confirmed that your clip is in fact 1080p?
When you create a project as I described in my earlier post, and insert a 1080p video clip into it as the first clip added to the project, that should set the project resolution at 1080p and give you a 1080p share option. What shows as the project resolution when you click on the Settings button above and to the right of the timeline?
Here's some other diagnostic tools that you might not have tried:
Safe Mode
You could also try booting up in Safe Mode (see Finder menu for instructions) and,
after everything loads, immediately shut down and reboot in normal mode. Then
see if iMovie works properly.
Open iMovie in a new library
Try opening iMovie in a new library. Hold down the Option key while opening iMovie and chose to create a new library in the box that appears. iMovie will open in that library. Create a project, put your 1080p clip into it, and see if you get a 1080p share option.
Resetting the System Management
Controller
Try resetting the System Management Controller:
https://support.apple.com/en-us/HT201295
Resetting the NVRAM
or PRAM
Try resetting the NVRAM or PRAM, although this issue would be fairly rare:
https://support.apple.com/en-us/HT204063
-- Rich