The sufferings of Christ (τα παθηματα του Χριστου). Subjective genitive, Christ's own sufferings.

Abound unto us (περισσευε εις ημας). Overflow unto us so that we suffer like sufferings and become fellow sufferers with Christ (2 Corinthians 4:10; Romans 8:17; Philippians 3:10; Colossians 1:24).

Through Christ (δια του Χριστου). The overflow (περισσευε) of comfort comes also through Christ. Is Paul thinking of how some of the Jewish Christians in Corinth have become reconciled with him through Christ? Partnership with Christ in suffering brings partnership in glory also (Romans 8:17; 1 Peter 4:13).