First of all, the adjacent tooth which is d 2nd molar also has deep caries, plz ask ur dentist to check it as well.
Secondly, the 1 st molar has abscess under d roots along with furcation involvement.
To save the tooth and reduce pain, copious irrigation during retreatment sould be done.
Also, Periodontal Flap Surgery with/without bone graft should be performed since the tooth has furcation involvement.
Next Steps
Please visit any nearby clinic to meet an Endodontist ( RCT specialist ) and also a Periodontist ( Gum Specialist ) who can rightly help u out.