Kieran Agard scored the 75th minute winner but the Pilgrims did have chances with Reuben Reid missing two excellent opportunities.
That defeat means they slip to 19th, two points above second from bottom AFC Wimbledon, going into next Saturday's final match at Rochdale.
"I thought we played well against a team who are going for automatic promotion," Sheridan said. "We well matched them.
"I didn't see any danger and in the second half we were taking the game to them. We just switched off, it's a straight ball into the box and it ends up in the back of the net.
"I don't think we deserved it, but that's life and it's gone now. Results have not gone our way but it's still in our hands with one game to go, so we focus on that one game and next Saturday at Rochdale.
"The players could not have given me anymore to be fair, I am proud of them.
"We went for the result and I thought we definitely deserved something out of the game and if we had got something it would have kept us safe.
"It's just one of those things, we just switched off and it cost us. I said it would probably go to the last game and it's a funny old league. True to my word it's going to go to the last game."