Cholesteatomas can take years to form. They usually form when there is a vacuum behind the ear drum which pulls it back and causes a sack to form at the top on the other side. After I had my first one removed (age 30) and my ear drum rebuilt - my specialist could gradually see my ear drum being sucked back (over years) by looking down my ear canal. It took nearly 10 years for a second one to form. He did think at one time about putting a grommet (hole) to prevent a vacuum but that never seemed to happen.
My original symptoms when I first had one were 1. loss of hearing (no pain) for about 1 month, 2. then stabbing pain (lasted about a week), 3. constant aching pain (like toothache), 4. then increasing dizziness (felt like I'd been drinking). When the dizziness became so bad I admitted myself to hospital and a CT scan showed the cholesteatoma. I've known people that have had similar symptoms but over several years rather than months.
My hospital didn't use an MRI originally as it couldn't see something as small as a newly formed Cholesteatoma. It has recently just upgraded its MRI so it can now see one.
Another symptom is you tend to get big smelly ear discharges if one's formed.
As the cholesteatoma forms on the other side of the ear drum the specialist may not be able to see it by looking down the ear canal. In my case it did make a small hole at the top of my ear drum which was spotted by my specialist.
I'm sure you will be okay - the main thing is that you are being monitored by your specialist.
Take care! - Steve (UK).