、。
Truth be told, I'd be lying if I said I didn't regret deciding on the spot.
Literal
Truth [gen-の] place, instant-decision [obj-を] not-regretting [if-I-say-たら] lie was.
実のところ ('in fact, as a matter of truth') is a reflective discourse marker opening a confession. 即決 ('instant decision / snap decision') is a crisp Sino-Japanese compound. 悔やんでない is the contracted 悔やんでいない ('am not regretting'). ~といったら嘘だった is a fixed introspective formula: literally 'if I said X, that would be a lie' = 'I can't really deny X.' It's a subtle, hedged way of admitting something. This kind of litotes is characteristic of reflective Japanese prose.