You have likely overcharged it. If you look under the hood, there should be a sticker that shows the capacity of the system. If it was cooling at all, it likely had more than half what is listed there onboard already, now plus the 18oz. you've added. At this point, the best thing you could do would be to bite the bullet and take it somewhere to be evacuated and properly recharged instead of guessing. You can destroy the compressor or blow out a seal if the system is overcharged. If it pops a seal when you're working on it, you could suffer serious injury, not to mention an expensive repair.
There are a lot of factors that can cause a system to not cool properly besides being low on charge.
-The condenser fins could be dirty or the car's fan(s) may not be coming on.
-A sensor or electrical control could be bad.
-The charge in the system may be contaminated with air or moisture.
-An internal fault could be present in the system.
-Your "blend door" may be not working properly, causing some warm air to mix with the cold.