Start 'convincing yourself' of the opposite!
The mind is a powerful thing and can respond to self-talk (internal or out loud). If you keep telling your brain that there IS something to worry about, then it will keep reenforcing that point. Keep repeating out loud; 'Its JUST stretch marks', 'its a spot, I always get them'.
Every time a negative thought pops into your head, counter it with a whole series of reassurances.If necessary write them down, everything that is reassuring - from the threads and from yourself. Read them out when a negative thought comes to you, over and over and then get on with something else. YOU have to take control of this, strongly, or it will control you. It's hard, in the early stages to retrain your mind like this, to respond more appropriately to minor symptoms or observations, but it will reap rewards longterm.
It takes effort to retrain yourself, and willpower to make sure you don't let the negative thought have the last say! Take it from me (I've had BC), I had to learn this myself, as every single niggle, twitch or spot became something sinister.