Argyle, unbeaten in six in all competitions ahead of Tuesday's FA Cup replay at home to Port Vale, took a 65th minute lead through midfielder Dominic Blizzard's 20-yard drive, which took a deflection off Michael Timlin.
Southend replied two minutes later as danger man Kevan Hurst latched on to a through ball from substitute Cauley Woodrow to claim his 10th goal of the season.
Plymouth central defender Curtis Nelson then hit the post with a shot in stoppage time.
Sheridan said: "It was a well-deserved point, we should have won the game and it's a shame that Nelse's shot came back off the post.
"Southend are a good team, probably one of the best organised and in-form sides we have played here, they have had some good results and are going really well.
"That said I thought we were the better team, so full credit to the players again.
"We had some great opportunities from free-kicks in and around the box in the first half but didn't do as well as we should have done. In the second half we really came out and got at them.
"The crying shame is that we didn't keep the lead for longer. If we had we would have probably gone on and won the game. Even when it went to 1-1 we were the team who were chasing.
"It was a tough game but the players really stuck at it and I think we have come a long way.
"With those unbeaten games we have got a bit of belief in ourselves and it was an excellent performance, it was just a shame we didn't win the game.
"We want to be greedy and we want to get three points because the games run out and we want to try and catch the teams above us."