Lit. no longer forbearing. See on 1Co 9:12 : lxx, Sir. 8:17. For Class. parall. Soph. O. C. 15; Elec. 1118; Eurip. Hippol. 844; Ion 1412. He means that his longing for some personal communication from the Thessalonians became intolerable.
To be left - alone (καταλειφθῆναι - μόνοι)
Implying, as we sent (1Th 3:2) and I sent (1Th 3:5), the previous presence of Timothy with him at Athens.