I had a fear of brushing my teeth which i have gotten over now but i have a cavity can i stop it progressing by brushing 2 twice a day or is it going to continue to decay?

You cannot stop the progression of a carious lesion that needs to be restored. Have your dentist restore it now, while it's small. Failure to treat will only make matters worse.

How do you know you have a cavity? Brushing cleans the bacteria off the teeth that causes the cavities and gum disease. See a dentist for exam and treatment. Your cavity, if diagnosed will only get bigger causing more problems.

You should be brushing 2x/day for 2 full minutes by the clock. You also have to remove bacteria from between your teeth daily from between your teeth where a t'brush can't reach. Cavities/gum disease are infections caused by not removing bacteria. Once cavity starts you cannot prevent spread. You need a Dentist to do that for you. Do NOT delay.