Castleford’s hopes of lifting the trophy for the first time since 1986 were ended with a 26-12 loss at Wembley
The West Yorkshire club were 12-6 ahead at half-time but Saints got back into it with James Roby’s contentious 42nd minute score.
Lachlan Coote hoisted a high kick which Mark Percival appeared to nudge forward.
The ball bounced marginally inside the touchline and Regan Grace, initially standing in touch, jumped up and palmed it back infield where James Roby raced over.
Referee Liam Moore awarded ‘try’ on the field but it was only confirmed by video referee Chris Kendall after countless replays.
However, Powell insisted: “That try, somebody had to go catch that.
“It looked like it went backwards. (But) I thought it then hit the line.
“There was a fair bit of doubt in there but we should have scrambled a bit harder from the inside there and we potentially stop that.
“We have to look at ourselves.”
Tommy Makinson scored soon after as Saints took the lead, meaning Powell lost a third final in his Castleford reign after the 2014 Wembley defeat and Grand Final loss three years later.
“That one (Makinson) down our left edge was really soft when (Jack) Welsby goes all the way across us,” he conceded, his side limited to first half tries from Lance Todd Trophy winner Niall Evalds and Jake Trueman.
“I thought we played well but we couldn’t find our way through.
“But I thought they defended really well.
“I’m disappointed but I’m proud we got here. We had a real go today. Physically we put ourselves on the line and they just got us.
“When we had good field position they (Saints) defended phenomenally well. Their scramble was great.
“You look at how many points they have conceded in Super League they do take some breaking down.
“We lacked a little cutting edge. It’s always hard when you go into these games and you don’t come up with the prize.
“It will hurt quite a lot in the morning I’m sure.”
Meanwhile, Powell revealed prop Suaia Matagi missed the game after falling ill on Thursday meaning Alex Foster - who was due to miss out - came in on the bench.