A late Florin Andone header gave Albion the win against bottom of the table Huddersfield, whose fate of relegation seems inevitable with ten games of the season left to close a 13 point gap from 17th placed Southampton.
Albion boss Chris Hughton said his side deserved their win having posed far more of a threat, particularly in the second half, but Siewert did not agree.
When asked if his side were beaten by Brighton's 'quality', he replied: "Quality? We lost the game because of a free kick. One situation where we were not switched on.
"You cannot allow that in the Premier League. It changes games. We leave with nothing in our hands and we definitely deserved a point here today. It is one situation which changes everything.
"I'm angry about it. Look at the supporters who came from so many miles away, supporting us for the whole season, and we go away with nothing in our hands because of not being switched on in a free kick situation. It is hard, I tell you.
"We have to learn that this kills you in Premier League."
Siewert said his side 'did everything' and did not look like a team 16 points behind their opponents.
He added: "They emptied their tank against Wolverhampton and today they did it again. I enjoy working with them everyday and they are working so hard. I see the progress but they have to understand that it kills themselves.
"I am angry because I want to continue and I want to get points. In the next games I am going for points. I am helping them so these situations won't happen.
"They (Brighton) are many points away from us but it didn't look like this. We did everything, we fought.
"We had to make substitutes. Stankovic was nearly dead. He emptied his tank once more today. In the first half, because of a knock, Hogg couldn't breathe so we had to substitute early and this is not easy in such a game.
"I knew that it would be difficult here. Their manager is doing a fantastic job but we lost because of a free kick and this makes me very angry to be fair."