If only my father had lived longer.

Literal

Father [subject-が] more long-life had-been [if-たら] [wishful-なあ].

~していたらなあ is a counterfactual wish: 'if only X had happened.' なあ at the end expresses wistful longing. 長生きする = 'to live a long life.' A poignant expression of grief.