Getting Out of My Swimming Pool Comfort Zone

I started to learn to swim back in March. Since then, I can do the basics such as the front crawl and back stroke. My problem is that I don't feel comfortable in water that I can't stand up and actually touth the bottom of the pool. That restricts me to staying in water that's 5 1/2 feet or less. I use to be afraid to backstroke in 3 feet of water and I got myself out of that once I learned to stand myself up while in the water. I have not made any progress since. I won't even allow myself to learn to tread water because I am afraid to even get in deeper water even though the ropes and the wall is there for me. I don't know what to expect. Does anyone have any suggestions for me? What happens if I try to get myself to a stand up position in deeper water? Will I start to go down? My goal is to be able to get into 12 feet of water and be comfortable, and then I want to start swimming in the ocean. But, I don't know what's holding me back. I know it can be done because Masters can do anything in the water. Any suggestions will be greatly appreciated. Thanks!