Cook said: "Three points come and go every week but games like this are for bragging rights and unfortunately we've let our supporters down.
"But I think over a period of a couple of months and even in the performance here I don't think our supporters could feel let down because they could see the lads doing their best and trying to do the right thing.
"Some things don't go your way in football, we've been beaten by Mansfield and we've got to take it on the chin.
"It's tough and, believe you me, the next few days will be hard work. When we look at the league we are top yet we still feel like we've been relegated."
On the sending-off, Cook added: "Gary wants to deliver so much and sometimes when it's not your day and it's not working it leads to that and we've got to make sure we manage that properly. Gary is a great lad and that's totally out of character for him."