Disclaimer: I do not own any Doctor Who characters or actors, even though that would be quite cool and I'd like David Tennant to do my grocery shopping/homework.
The first time Koschei – not quite the Master yet – almost said sorry was when they were children under the big red sky of Gallifrey, and he accidently broke Theta's favourite toy. His big eyes had filled with tears, and Koschei had tried to remember his manners but had felt too awkward of the boy he'd only just met, so he said nothing as his playmate cried.
The second time Koschei – almost the Master now – almost said sorry was when he was in his teens, about to leave the red sky far behind, and Theta had finally found the courage to kiss him. He'd slapped him round the face, and youthful pride and the resounding drums had made Koschei swallow any guilt he felt at making his friend cry and again and walk away.
The third time Koschei almost said sorry he was the Master, high in the blue sky with a bullet lodged in his chest and Theta's – no, the Doctor's – arms holding him up. He almost apologised for the death of millions and the third instalment of tears from the Time Lord, but humiliation and pride made him mouth 'I win' instead.
The fourth time the Master almost said sorry was at the end of time and the cause of the drums, his insanity, became clear and he turned his hatred on the people who'd caused it. For a moment he saw the tears building in his mentally and physically broken Doctor and nearly apologise for turning down so long ago what he'd wanted all along – what could've saved both of them – but then he faded from view and the moment was lost.
The fifth and final time the Master – now feeling more like Koschei again – almost said sorry he saw the Doctor, a new version that almost looked like his Theta but not quite, with a red-headed woman across a crowded street. He nearly walked over and asked for forgiveness; to stay with him and travel time and space like they'd always planned. But he understood that this would only lead to more tears so he walked away without turning back, a blue sky at his feet and finally being the Master of his future.
And that – he hoped – meant no more sorries.
Well, hope you liked!