The Seagulls were condemned to a fifth defeat in their last seven home games by second-half goals from Pawel Abbott and Reuben Hazell.
The uninspiring performance triggered a chorus of boos afterwards and Forster conceded that the players feel as unhappy as the supporters about the situation.
Veteran striker Forster said: "For 18 months we've been a team who are struggling and we have to knuckle down and get on with hard work.
"The fans are unhappy and so are we. There was a sombre atmosphere in the dressing-room."
He added: "We must grind out results, our squad is capable of going on a run - the strength in depth we have is fantastic - but we can't find that form at the moment. It's all very well to say things are not going for us but we have to be better than that. We must come out on top in these tough encounters."