For the second year in a row, Leinster failed to add the RaboDirect PRO12 title to their European success, missing out by the narrowest of margins to Ospreys in a thrilling 31-30 defeat.
Twelve months previous, Cullen and co had defeated Northampton to be crowned kings of Europe but were then downed Munster as they pursued domestic honours.
Indeed, Leinster have lost the last three Celtic League finals - also going down to Ospreys in 2010 - and their place in history as the first side to win the European and domestic double continues to elude them.
So while you may think a third Heineken Cup win in four years tasted sweet, warhorse Cullen insists his summer was unpalatable.
"After the Heineken Cup final [against Ulster] the lads took it easy, we didn't celebrate," said 34-year-old Cullen.
"We were keen to try and push on and do the double and you could see the look on the players faces how much it meant to them.
"Three years ago we lost to the Ospreys and lost to Toulouse in the Heineken Cup semi-final so the season fizzled out - it's not like we weren't trying to win the games but it just didn't happen.
"Two years ago we played Munster having beaten Northampton in the final of Europe the week before but playing Munster at Thomond [Park] is never an easy proposition.
"They're all different games but we just didn't really spark and then losing to the Ospreys, it could have gone either way.
"We were up in the second half and looking like we were going to win the game but both games there were very small margins between winning and losing.
"We were missing a few players and you wonder how much they made a difference.
"[Winning the double] is like the Holy Grail, it has never been done before. It's difficult, if it was easy we'd all be doing it.
"I found myself watching the game the following week trying to pick the bones out of it.
"We are European champions but when you've got the opportunity to do something special and don't quite get there it's hard.
"At the end of the game, it was when I saw the reaction of my teammates - that was what made it hard."
It's a measure of the sustained success enjoyed - and expected - that last season can be viewed as disappointment. Only Leinster are able to maintain their challenge on multiple fronts when it comes to juggling RaboDirect PRO12 and European commitments.
Last season Edinburgh made history by reaching the last four of the Heineken Cup but paid the price in the PRO12, finishing in 11th place while Glasgow were fourth but saw their European ambitions suffer as a result.
Head coach Joe Schmidt is largely keeping faith with the bunch that fell at the final hurdle last season however - 19 players were resigned while only a sprinkling of Southern Hemisphere talent has been added to the squad in the shape of centre Andrew Goodman, prop Michael Bent and South African second-row Quinn Roux.
Leinster will therefore continue to rely on their tried and tested conveyor belt of young talent with Dominic Ryan, Rhys Ruddock and Ian Madigan all set for leading roles this term.
And Cullen revealed that the exuberance of youth is what keeps his ageing limbs going.
"I'm 34 and I'm towards the end of my playing career but it's watching the youngsters coming through and taking up the mantle for the club that's exciting and keeps you motivated," he added.
"Some of them will thrive and some of them of them will fall by the wayside.
"We tend to rely on our home grown players and just add a bit of experience here and there but it's always exciting to see how the new wave will shape up and if they're up to the mark.
"A couple of faces leave and a couple of faces come in but we are relying a lot on the new academy players coming through and it's exciting when you see that."
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