Congrats on the weightloss. As a former "big boy" who used to be obsessed with coke, pepsi and all that other unhealthy shit, I can sympathize with you.
Now in regards to caffeine and tolerance, you might have developed it. Long term ingestion of coke (sugars) at that high amount can lead to adrenal burnout. I know that some people consider this a myth, but it can happen especially if you eat a lot of refined food (like you say you did). Going cold turkey is rough, but sometimes the best way to go.
What I would suggest to you, from my experience of cycling stims and thermos for years, is to try and take the caffeine on an empty stomach. Some people get a major buzz off of 5-hour energy, some don't get shit from 10 cups of coffee.
If you are interested in a good "stack" look into making an E/C/A stack. It will definitely help.