The Swiss captain, who will lead his team in their opening Euro 2012 qualifier at home to England on Tuesday, has scored 40 goals in 78 games but the last of those was one year ago in a World Cup qualifier away to Latvia.
He failed to hit the target during the World Cup, when he was troubled by an ankle injury, and missed a penalty in Friday's goalless game which earned Frei a chorus of whistles from the crowd in St Gallen.
"I feel fine, I have scored lots of goals for my club in the last few weeks so my self belief is absolutely intact," Frei told Reuters.
"My self belief has never been lacking, sometimes I have a bit more, sometimes a bit less," added Frei, whose goals at club level have helped FC Basel qualify for the Champions League.
"But when you work hard, when you train, when you score goals regularly in training, then you know you are going to end up scoring in a match."
Frei declined to criticise the St Gallen crowd.
"A fan pays a lot of money for a ticket, so he has the right to vent his displease," he said.
"It is up to the fan to decide in which way he does this, some players find it not correct that someone whistles when he is not satisfied with the team.
"I can certainly say that it does not help the team when they jeer.
Switzerland have managed only four goals in their last 10 games, including just one at the World Cup where they beat eventual winners Spain 1-0.
"This is not the best day to be worrying about our scoring record," coach Ottmar Hitzfeld told reporters, hinting at a cautious approach.
"We want to win the game but England are the clear favourites.
"We must try and find a balance between our performance against Spain, where we defended very well, and going forward."
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