The odds are it isn't cancer. Only about 5% of thyroid lumps are cancer. The rest are benign and there is usually no way of telling which is which without a biopsy. And the doctors have no reason to lie. If they say it's probably not cancer, then...it's probably not cancer.

Also, even if it is cancer, and the odds are VERY much against it, you're still not going to die. i had thryoid cancer 6 years ago and for the most common forms of thyroid cancer, the survival rate is close to 100%, especially for those under about 50. I've joked that the biggest threat it was to my life was that I got such a shock hearing the word "malignancy" that I could have walked under a car. I've read studies on it and...there were two separate ones that ended with "we also wanted to study the effect of whatever we were studying (I think one was radio-iodine therapy, can't remember what the other was) on death rates but we had to drop that because we couldn't find enough people who died of it to make stats."

I mean, any chance of any form of cancer is terrifying, but the odds of death for you are very low indeed. Worst case scenario, you need your thyroid removed. Which yes, is a serious operation and some people have problems getting their thyroid levels right after it (I didn't and from what I've heard, most don't but something like one in six do). I was off work for a month in total and then just...got on with my life.

I will also say that I had that lump on my thyroid for probably at least five years before I was diagnosed and was never fatigued. It's probably more likely that the fatigue is related to the possible hypothyroidism than to any risk of cancer.