Try not to worry too much. As I kept reciting to myself before I was diagnosed "thyroid nodules are common. Thyroid cancer is rare." The odds are that it's completely harmless.

And even if the worst case scenario, where it is cancer, well, assuming your username includes your date of birth, then at our age, the most common types of thyroid cancer have a survival rate, I think over 99%. It doesn't even have a Stage 3 or 4 unless you are over...I think it's 55 now. And they don't use ongoing treatments like chemotherapy or radiotherapy.

If you do have cancer, and you probably don't (I think about 95% of nodules are benign), the odds are they will remove your thyroid and...that will be the cancer gone. Considering how large it is (though mine was 105mm, but that was...well, googling you can't even find much comparative!), they may want to do radio-iodine treatment afterwards, to reduce the risk of recurrence, but that is basically just swallowing a tablet. It's a nuisance because there are a lot of rules around it but it's not going to make you sick or weak or anything and it's just a once-off.

I was diagnosed with thyroid cancer in November 2019. I worked until the Christmas holidays, had my operation on the 6th of January, woke up to hear the surgeon saying, "we got it all," was sick for a couple of days due to the anaesthetic, then went back to work in early February. Apart from concerns about possible recurrence, it wasn't really any worse than I'd imagine having say a gallbladder or appendix or tonsils removed would be.

I had the same situation as you with regards to not noticiing it. It was there for at least 5 years by the time I did because I can now see it in older photographs from 2015 or so.