Hello sir.. what your dentist said is correct... you need to go for root canal treatment followed by capping for permanent solution....

enclosed x ray clearly shows its as a borderline case ...as just decay has approached the pulp so to avoid the root canal expense ..doctor would have tried normal filling but while treatment its almost near to pulp so hence forth doctor has taken a good decision to go for root canal treatment one shot to relive it ...rather than doing filling then again doing root canal after days ...good decision of doctor

The cavity is deep and is involving the pulp. So you will have to get the pulp extripated. RCT is the solution. Donot delay the root canal, get it done as soon as possible.

This looks like second tooth from midline having dens in dente which means tooth us having fold in itself..secondary cavity happened later..RCT will solve your purpose..please go ahead...