Up against relegation rivals Staines, Bracknell appeared to heading for a second successive win as they led by 11 points with 10 minutes of the match
In addition, Staines had two men in the sin bin, but they still fought back to snatch a 37-31 victory and leave Bracknell with just four games to save themselves from relegation from National Three South East.
"We were 20-11 up at one point and we let them back into the game. We threw it away ourselves," Leishman told the News.
"We gave away too many penalties and just didn't deal with the referee in the right way."
Bracknell were made to pay for conceding a number of penalties as Staines battled to their second win of the season over the Lily Hill men.
The only consolation for Leishman's side was that Westcliff, who sit a place above them in the table, were also beaten.
Nevertheless, Bracknell coach Leishman acknowledged that the next two matches against London Irish Amateur, who are two places higher, and Westcliff, are pivotal.
"We have four games left in the season and we have to win the next two to see where we are, if we don't manage to do that then I think our fate will be sealed," he admitted.
"We targeted Staines as a game we had to win and then we could maybe lose one of our others but now we simply have to win our next two matches."
Bracknell are without a match this weekend but a few of the first team players will get a run out for the 2nds.
They will be hoping to earn a first team place when Bracknell return to action on March 24 with a trip to London Irish (3pm ko).