Srna and substitute Douglas Costa twice put the home side ahead in Wednesday night's Champions League last-16, first-leg tie at the Donbass Arena but goals from Robert Lewandowski and Mats Hummels, the latter coming three minutes from full-time, earned Dortmund a hard-fought draw.
"It could have been better and obviously this is not the result we would have wanted," Srna said.
"We were in front twice, but on both occasions we let that lead slip away, four minutes before the end of the first half and at the end of the match we gave away a silly goal.
"At this level you can't get away with making such mistakes. We have to look forward, it's just a shame that all our hard work has been undone.
"We have got three weeks to prepare for the next game and I am sure that we will play better in the return leg.
"Two-all is not the result we wanted and we have to win in Dortmund, but the tie is still open and we will be doing everything to try and get the result we need. Borussia were fitter than us today, but that's not surprising as their winter break finished weeks ago."
Shakhtar boss Mircea Lucescu was disappointed by his side's defending.
He said: "It's a pity that we didn't achieve our goal, which was to beat Dortmund, but quite frankly, I have to admit that the end result was fair.
"Our opponents once again proved they are a very organised team who are well prepared physically. They played aggressively and tried to attack whenever possible, creating a lot of problems for us.
"We couldn't keep the lead we had twice established and we made some errors. That is inexcusable.
"It's a 2-2 draw, but we still have some time to work and prepare" [for the second leg].
"We made schoolboy mistakes today and now we have to work hard. We talked a lot about set pieces while we were preparing for Dortmund, but they still used one of their big strengths to score."