The three repetitions of the phrase "Now too late" are perhaps included for meter's , cadence's or poetry's sake. But they occur in a progression of decreasing relevance. The words are perfectly fitting in "Now too late, I know that we were wrong to part". The connection of "Now too late" is a little more vague with "we can't go back and start anew". Then "Now too late" is completely impertinent as a phrase prefixed to "I've thrown away my chance of happiness with you". Because the misfortune in that throwing away was in the fact that it happened at all. Doing so any earlier would still have been the same misfortune of throwing away that chance. So the third "Now too late" is totally impertinent to the phrase to which it is prefixed. In other words, as the lyrics stand, it is as if we were saying "I should have thrown away my chance of happiness with you sooner." But what she is really regretting is that she threw it away at all, not a matter of bad timing in the case of that throwing away.