Trinity have dropped out of the top four with two games remaining in the Betfred Super-8s.
Two victories would still secure third place and a semi-final at Leeds Rhinos, but one win may not be enough to qualify for the play-offs.
Trinity visit fourth-placed Hull on Thursday before a home game against Wigan Warriors, who are third, nine days later.
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Hull and Wigan are both a point ahead of Trinity, who suffered a heartbreaking late defeat by St Helens last week.
“We just have to take it week by week now,” Fifita said of the upcoming challenge.
“We are still in a great spot. Everyone expected us to be in the million-pound game, we were one of the favourites to be in the bottom two so we’ve proved everyone wrong.
“Don’t write us off now. I think Saints are the best team in the comp – they beat Cas and they have a huge forward pack – and to stick it out with them was a great effort. We are going to be doing it the hard way now, but that’s the way we love it here.”
Fifita added: “Hull is our first hit and we will go there on Thursday night and we are not going to hold anything back.
“We will take it straight to them and we have nothing to lose. We had nothing to lose last week and that was our mentality going into the game.”
Trinity were the better team for 78 minutes against Saints and Fifita said their performance will send them into Thursday in good heart.
“You saw in the second half, when we were down to 12 men [with Dean Hadley in the sin-bin], how much work the boys were putting in for each other,” he added.
“What we have built here as a club this year has been great and all the boys here, we are all brothers.
“Tyler Randell has come in and settled in pretty good and he has made a big impact. It would have been nice to get a win last week, but we played well for most of the game and we just let it go at the end.
“It wasn’t one person, it was a whole team effort.”
Fifita stressed he – and his team-mates – still have full faith in full-back Scott Grix, despite his error which led to Saints’ winning try.
“We don’t blame him, no way,” Fifita insisted. “The second half lasts 40 minutes, we were all out there so it’s pretty much everyone’s fault.
“There was a lot of positives to take out of that game. We will look at the video this week, put that game aside and go to Hull and take it to them.”