• The speaker describes a different scenario, now. He was once angry with his "foe" (a.k.a. his enemy), but didn't tell him about it.
• Since the speaker did not talk about his anger ("I told it not"), his anger got bigger and bigger ("my wrath did grow").
• You know how, when you keep something bottled up inside, it tends to make that feeling more intense and overwhelming? We're guessing that this is what's going on for the speaker here.