The Welsh club had won back-to-back games before their Premier League trip to Anfield, but were brought back down to earth in the final top-flight match of the year.
Alberto Moreno gave Liverpool the lead in a lacklustre first half and Brendan Rodgers' men raised their game in the second period to sweep aside the Northern Irishman's former club.
Liverpool were given a helping hand by Swansea, though, as Lukasz Fabianski's attempted clearance struck Adam Lallana and looped into the back of the net. Jonjo Shelvey also scored an own-goal after Lallana struck again with a fine individual goal.
Gylfi Sigurdsson had given Swansea a lifeline when he made it 2-1 a minute after Lallana's first goal.
And Monk knows his players will rue their sloppy errors.
"We played OK, had most of the possession and we created chances, but when you make that many mistakes any game you're going to come under pressure and make life difficult for yourself, especially at Anfield," Monk told Sky Sports.
"We made too many mistakes to warrant anything from the game."
Monk, though, stopped short of singling out goalkeeper Fabianski for criticism following his mistake.
He added to BBC Sport: "Lukasz Fabiasnki has been fantastic this season but every player makes a mistake. You lose as a collective.
"We had the most possession, we created chances, but the mistakes we made cost us the game. You get punished at this level."
Shelvey's disappointing return to his former club also saw him involved in a controversial moment when he caught Emre Can in the face with his arm late in the first half.
But Monk said: "I haven't seen the situation with Jonjo Shelvey so I can't comment."
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