Although Wales posted a best World Cup performance for 24 years in New Zealand last autumn, reaching the semi-finals, it could - and possibly should - have been so much better.
"We were a couple of minutes away, a point away from the World Cup final, and who knows what would have happened then?" Wales scrum-half Phillips said, ahead of Sunday's Six Nations clash against Ireland in Dublin.
"You start thinking about 'what ifs'. The feeling looking back on it is one of real frustration.
"Yes, we did well, but we went out there wanting to win it, and it was frustrating going so close.
"Maybe this Six Nations campaign is all about getting those frustrations out, doing well and winning. It's a tough opening game, but hopefully we can perform and gain some momentum.
"If you do well and win your first game, then it creates momentum.
"We are itching to get out there on Sunday. It's about keeping a cool head, going over there and performing, and then things will hopefully look after themselves."
Wales dumped Ireland from World Cup contention at the quarter-final stage in Wellington, and 60 times-capped Phillips recalls it as a benchmark performance.
"We need to create pressure and take our opportunities," he added.
"We made very few mistakes in the quarter-final - I think we only made three handling errors - and we were also very disciplined, which is what wins you international games.
"We need the same sort of formula on the weekend. We have got to be squeaky clean and make sure we are accurate, and I am sure with the players we've got in the team that we are going to create opportunities.
"The last two games we've played against them, we've beaten them, so we have got to be confident from that fact."
Both teams will be without talismanic players this weekend. Wales start Six Nations life after their retired, record-breaking wing Shane Williams and Ireland face a whole tournament minus injured captain Brian O'Driscoll.
"Shane was an absolutely unbelievable player," Phillips said.
"To score a try in his last international match like he did, he's like the Elvis of rugby, really, although not with his singing!
"Shane and Brian O'Driscoll are legends of the game. Brian, like Shane, pops up and scores vital tries, which he has done so many times over the years. He oozes confidence and brings confidence to the team."
But while those two rugby superstars will undoubtedly be missed, Sunday's match is likely to be another Ireland versus Wales battle dominated by two outstanding back-rows.
"The best teams in the world have the best back-rows - that is where the game is won and lost, really," Phillips added.
"We were outstanding there at the World Cup, and we are in a very healthy position at the moment in terms of our back-row strength in Wales.
"But it is also Ireland's strength, so it is going to be a big contest there. We have prepared well, so hopefully, we can come out on top."
And if Wales can achieve a rare Six Nations win in Dublin, it would be a perfect way for coach Warren Gatland to celebrate his 50th Test in charge since being appointed four years ago.
"I have probably played my best rugby under Warren Gatland," Phillips said. "He has given me a lot of confidence, and I have got a lot of respect for him.
"He seems to get the best out of players. The first squad meeting when he came in as coach, everyone was on the edge of their seats listening to what he said.
"And when you turn up for training, you know you have got to perform, and you know you have got to play well every game. He's a top guy - you want to win for him."
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