First of all. DON'T PANIC!
And even before that - make a nandroid backup! Go in CWM and make a full backup.
You have recovery working so you should be able to load System on it ok.
I am not an expert but let me try to guide you step by step.
1. Make a backup!
2. Download a rom on your computer. MAKE SURE ITS FOR YOUR MODEL # of LG G2
List of roms here
3. Make sure you have over 70% battery
4. Go into recovery CWM
5. Use Wipe > Swipe to Factory Wipe
(go back on bottom right corner)
6. Go Advanced from HOME screen of CWM
7. Click ADB sideload
7b. (I assume you have the drivers loaded. If not make sure ADB is loaded in windows and device manager shows it. Download basic ADB package so you have "adb.exe" file in it)
8. put your rom in the same folder where you have "Adb.exe"
9. Open command prompt on windows. Goto the folder where ADB.exe is. Then Type "adb push (filename) /sdcard/(filename)
". Example, "adb push rom.zip /sdcard/rom.zip
10. Wait until it finishes its transfer(a message will appear saying the filename, file size and the speed of the transfer per sec when it finishes)
11. Wait for a while or reboot your cwm so that the filename will appear
12. just reflash the file in CWM (Install > choose file). Wipe after again.