I texted a friend of mine that does software development for Apple and he was able to help me get past this problem. I was experiencing the same symptoms: upon starting my iMac, the progress bar would load underneath the Apple logo, and it would just sit there and never progress any further. Here's what helped me solve the issue without having to reformat my drive or reinstall the OS.
With the computer shut down, turn it on and hold down Command + S to launch a command shell. Give it a few minutes until it doesn't load anything else on the screen.
Type the following commands, pressing enter after each command:
mount -uw /
See if this boots you back into the system. If not, try this order:
mount -uw /
mv /Library/Extensions/*.kext /
mv /Library/LaunchAgents/*.plist /
mv /Library/LaunchDaemons/*.plist /
I was able to get back into my system, but I noticed if I shut down and tried to turn it back on, the problem still persisted. According to my friend at Apple, he was 99% sure it was third party software. So I uninstalled each new program I had installed recently one by one until I narrowed it down to which one I thought was causing my particular issue. My computer appears stable now, and it may be because a program I was using isn't updated for the latest High Sierra update, and the incompatibility caused something weird to happen on startup. Definitely not 100% sure that's what happened, but between using the commands above to get me back into the computer, and uninstalling things one by one and going through this process over and over until I narrowed it down, it solved it for me. Hope this helps someone else!