The 21-year-old thinks that the Saints were made to pay for a slow start to the game.
"We didn't start the game well. Tactically we weren't right and we had to have a little change midway through the first half," Ward-Prowse told club's official YouTube channel.
"They started well and we didn't. They punished us for that.
"They had a lot of the ball and space – especially out wide, which is why we had to make the tactical change.
"But we're disappointed in ourselves because it was a game we felt we could win and unfortunately we haven't.
"The start to the game is always important. They started better than us and had a bit more momentum.
"They got a bit of a lucky goal and that gave them the confidence."
Ward-Prowse also expressed his disappointment on missing out on bragging rights in the south coast derby as the Saints lost to the Cherries for the first time since 1958.
"Things just didn't go right for us here. We're disappointed and we've just got to try and put it right on Saturday," the midfielder added.
"It's a game we felt we could win and we're frustrated because we wanted to have bagging rights, but it wasn't to be.
"We've got a few days now until the next game and we've got to try and put it right."