Baron Stevenson, who headed HBOS's board from its creation in 2001 until it was swallowed in a rescue takeover by Lloyds TSB in 2008, told the inquiry: "I feel awful. I was chairman of the board. I feel responsibility. There are very few days I do not think about it."
But he frustrated the inquiry's chairman, the MP Andrew Tyrie, by denying the Edinburgh-based institution grew too fast. He said: "All over the world banks were caught like HBOS."
As the session ended after more than three-and-a-half-hours, Mr Tyrie told Baron Stevenson: "Some witnesses have been very frank with us and it's been clear to us from their evidence that they have been looking the reality of this catastrophic series of events in the face, and some haven't.
"You've been evasive, repetitive and unrealistic."
Baron Stevenson, educated at the Perthshire public school Glenalmond, had told the commission that HBOS's demise was largely caused by the gumming up of money markets in 2008.
"We failed, along with the rest of the world, to anticipate the protracted closure of wholesale markets," he said, before suggesting HBOS could have survived.
"Had wholesale markets not completely closed, I think events would have been different. It was the closure of wholesale markets that did for us."
After being challenged by Mr Tyrie on the competency of decisions by Bank of Scotland's corporate division to make loans, some of them for billions of pounds, as the market reached its peak, the peer said: "There must have been some lending that was incompetent and a lot that was competent."
He attributed the enormous scale of losses to the "niche" nature of Bank of Scotland's lending.
The corporate banking division was led by Peter Cummings, who has been fined £500,000 by the FSA and banned from working in financial services.
The combined Lloyds Banking Group required a £20 billion capital injection by the taxpayer.
Baron Stevenson insisted the governance of HBOS "was rather good", and its structure as largely autonomous divisions "was working rather well".
He added: "What I do not agree with is that it was run in such a way to make [its demise] an inevitable consequence. Quite the reverse."
The commission published a letter from Baron Stevenson to then Financial Services Authority chairman Sir Callum McCarthy from March 2008, six months before HBOS failed, in which he insisted: "We have faced into the need to be boringly boring for the next year or two". He added: "HBOS in an admittedly uncertain and worrying world is in as secure a position as it could be."
Baron Stevenson yesterday continued to criticise the short-selling that led to HBOS's share price plunging in March 2008.
He told the commission: "I am not convinced it was the purest of process by the people who did it."