Trossard was excellent for Albion against United and hit the woodwork on three separate occasions.
The Belgian international cracked the post twice in the first half and then rattled the crossbar in the second period, with United goalkeeper David de Gea well beaten each time.
A penalty deep in injury time handed Manchester United a victory after a late Solly March header and an earlier penalty from Neal Maupay looked to have secured a deserved point for Brighton.
Adam Webster and March also hit the post and bar as Albion became the first ever team to hit the woodwork five times in a Premier League match.
Graham Potter's unlucky team were also controversially denied a penalty by VAR after Aaron Connolly was sent tumbling by Paul Pogba shortly after the break.
“I’ve never experienced a game like that to be honest,” Trossard said.
“I think we were the better team and created a lot of good chances, so to lose in the last minute is really difficult to take.
“I hit the woodwork three times but it was one of those days. Hopefully next week those chances will go in, but it is a tough one to take at the moment.
“We can take plenty of positives though and we have a good chance on both Wednesday and Saturday to get that winning feeling back.
Trossard admitted the late penalty for United, after Maupay handled in the box, was hard to take but he believes this Albion team proved they are capable of competing with the very best in the division.
“It’s always hard when you concede a penalty so late. But we played really well though and we have to move forward with that.
“Our performances so far this season show our development as a team and a club,” he added.
“We want to move forward, play attacking football and make it difficult for our opponents whoever we are up against.
"We’ll keep on going with that mentality we have and keep trying to win matches.”