I always get the unconscious "what if…" thoughts in my head. I know it is hard to stop them. Even when I have no symptoms, now and then throughout the day, each day, that voice goes "what if…" and I usually stop concentrating on the environment and lose track of what's going on.
I also always google about random cancers - even when I don't have any symptoms that are worrying me. I know I shouldn't. But I always check the stats - I actually use the statistics to try and reinsure myself that I am fine. It works sometimes… but then I start to tick of the risk factors I have and start becoming anxious.
The important thing to realise is this: during the time we spend concentrating on "what if…" and checking out all the risk factors, we are essentially wasting our time worrying about what could be, for the most part, nothing serious. This is easier said of course - being guilty myself, I have no room to talk. But of course, we should start to focus on the positive things. Every time you think of a negative "what if…", follow the thought up with a positive "what if…". So if you worry about it being cancer, think to yourself "what if it is nothing serious". Maybe this will help.
It sometimes helps me. And remember you are young. And also if I was you, I would stop the smoking. I have been cessated for a little over a year and I still get anxious about getting cancers because of the damage I have done due to smoking. I probably may never get over the anxious thoughts due to the smoking. And so this actually helps me in working towards a better lifestyle. Because of my anxiety, I not only never want to touch a cigarette ever again, but I always try to avoid second hand smoke also. So in a way, you could use the fact that you have these polyps as a motivation source for improving your lifestyle.
If you concentrate on improving your lifestyle, whether that be diet, quitting smoking and drinking alcohol, starting regular exercises, etc., not only will your anxiety decrease due to the fact that you are decreasing your risk, but your anxiety will reduce due to the fact that you will be keeping your mind active on other things.
I often enjoy learning about stuff - and I learn by reading and watching lectures. During my periods of learning, my anxiety levels are really low and I just forget about my worries. So do something you enjoy. I know that anxiety can often make you not enjoy stuff - but if there is something you enjoy, go out (or stay in if it is an indoor activity) and do it.