Don't worry, the likelihood of you having cancer of your stomach is ridiculously small.
I've had GERD since I was a baby and Rennies rarely helped me. It might be worth trying Gaviscon or you can buy Zantac over the counter.
If your diet is bad then that can make your stomach protest. To have GERD you don't necessarily have to have acid or anything in your mouth.
Maybe try cutting out caffeine such as tea, coffee, sodas and do you drink alcohol? That can cause pain too.
It's definitely worth you going to your GP though, they can really help you.
Start by trying gaviscon 20 mins after each meal and then before bed, go for the liquid rather than the tablets because the liquid creates a barrier at the top of your stomach to help stop stuff coming up.
Also anxiety can cause those chest pains too. The human body is very clever but very frustrating at times