The former captain began the RBS 6 Nations as Wales' first choice hooker but he was left out of the squad following the 30-22 loss to Ireland.
Richard Hibbard went on to make the number two jersey his own, while Ken Owens produced a string of impressive performances from the bench.
"It has probably been the toughest couple of weeks of my career," Rees told the Western Mail.
"I was starting against Ireland and then not to be involved in the squad at all after that was disappointing.
"I was frustrated with my own performance in the Irish game and the team's display because we gave ourselves too much to do to catch a quality side like that.
"But, credit to Richard, who has been playing some of his best rugby with the Ospreys.
"He was given an opportunity and he has taken it with both hands.
"The strength at hooker in Wales is as strong as I have ever known it.
"Richard and Ken now have had more experience over the last couple of years in the Welsh set-up.
"That is good for players because you want to be kept on your toes.
"If you go off the boil for a bit then you won't get selected and that is probably what has happened in my case."
But the Scarlets hooker insists he still has plenty to achieve on the international stage.
Rees missed the 2011 World Cup due to neck surgery and he is targeting the showpiece tournament in two year's time.
"I want to play in the 2015 World Cup in England," said Rees. "What has proved the most frustrating thing in my career is missing the 2011 tournament in New Zealand with a neck injury.
"I knew in the build-up to the tournament we were going to be good so missing out on that was devastating. You only have one career and international rugby is where you want to be playing.
"I have three or four years left in my career and want to finish on a high and that includes Test rugby.
"I feel as if I have a lot to give to Welsh rugby and hopefully hard work and dedication will get me back in that squad.
"It is about knuckling down and trying to get back in the team. I just have to come back stronger and prove myself."
Rees will leave the Scarlets to join Cardiff Blues in the summer. The west Wales outfit are still in the play-off race and he is determined to finish his Scarlets career on a high before beginning the next chapter in Cardiff.
"We have got four games left for the Scarlets and hopefully we can make the play-offs," said Rees.
"We are in fifth spot at the moment just behind the Ospreys and we have a good run-in.
"Personally I am coming to the end of my nine years at the Scarlets and want to finish on a high.
"We have had some great times like reaching the Heineken Cup semi-final in 2007.
"But it has been a long time since we won some silverware with the Scarlets and that would be great to achieve.
"The Scarlets was where I started to make a name for myself and who gave me an opportunity after the Warriors were disbanded.
"I am probably the last of the old guard and there is a good young nucleus coming through who hopefully can aspire to greater things over the next couple of years.
"It will be a sad moment because the Scarlets have always been a big part of my life.
"But I am also looking forward to the last chapter."
"One of the disappointing things for the Blues over the last couple of years has been the number of senior players they have lost," said Rees.
"When you lose core players like that it is tough to replace them. I was keen to see what players they were going to bring in and they have recruited well.
"They have done well with bringing back Gethin and this year players have come through like Rhys Patchell and Josh Navidi who have been outstanding.
"It is about having that balance of the core senior players and the youngsters coming through. That is what the Blues need in big games."