Hello mam ..    your history seems  you are having irreversible pulpitis...   Irreversible pulpitis is nothing but the debtal caries which affects the inner pulpal layer of your teeth ... Root canal treatment is the only option for this....

Hello Yes sometimes cavities are hidden and are only detected in x ray Go for a rct

We can only see the upper part of our tooth... The adjacent sides and root part can only be visualised properly in an xray. The pic of your xray would have been helpful. But as suggested by your dentist, get root canal done as it is the treatment of choice. Your symptoms say that tge infection has reached your pulp, i.e. the blood vessels in your tooth. So a root canal treatment can only help you get rid of the infection. Dont delay much as the infection may progress further.

Yes what he has said is true,because it happens many times that we(being human) can't see any cavity from outside and then comes the role of technology and science and so X-ray shows everything and as u have mentioned that u haven't been able to sleep in recent times is because of the cavity that has extended upto the living part of the tooth called pulp or nerve which can be treated with RCT only.