Miyoko had been carrying a one-sided love for a very long time. She must have been suffering greatly.
Literal
Miyoko [topic-は] quite long time one-sided-love [object-を] continuing was. Surely chest [object-を] paining was thing [probably-だろう].
片想いを続けていた describes sustained unrequited love. さぞかし means 'surely, no doubt.' 胸を痛めていた means 'was hurting in the chest/heart' — a common expression for emotional pain. ことだろう expresses sympathetic conjecture.