First, you may want to make sure you used the right rom version. I'm not sure what the SGN2-L900 is, but that's too different to be a typo for the SPH-L900, which is what this forum is on. I tried a quick search, and no phone variant came up. In addition, trying to flash a rom from a different phone variant can be very bad, and can brick a phone. Luckily, since you can still boot the phone and get into recovery, not all lost.
Plus, when you're downloading a custom rom, such as Cyogenmod, you should take note of the GAPPS file to be used. CM11 uses a different GAPPS files than previous versions such as CM10. Plus, usually you want to flash GAPPS after the CM rom. If you forget to flash it, you can do it later; it's just inconvenient
Titanium Backup is purely an app backup. It's not a ROM backup; this is what a nandriod is used for and one of the tools in the custom recovery. Plus, it is not recommend to use TB to restore apps when switching roms. It should be fine if just updating a rom, such a CM nightly.
If you're having no luck, I would suggest finding a stock tar file, and odin the firmware. This will remove root. But if you've had your phone over year, it'll already be out of warranty. Thus, when you go to re-root, it won't make a difference if you trip Knox warranty void. Just make sure you use the correct ROM for you phone. The odin the can be slightly challenging to find, but a doing a google search should be sufficient. I find version 1.85 or 3.07 work well.