The Spanish playmaker arrived in 2013 from Sevilla, but played 12 games in three years and spent a lot of time on loan.
Alberto is now a key player with Lazio, and speaking to France Football, in quotes translated by Sport Witness, he said: "I think it's a little bit my fault.
"I didn't do everything necessary to be at my level in terms of work and focus.
"What I should have done at 20, I was only able to do it at 25.
"It's a shame, I lost a few years. I'm 27 now. I'm trying to make the most of it and I think there's a lot of time left to continue to enjoy myself.
"I left Spain quickly, and especially a context, at Barcelona's B team.
"At Liverpool, I adapted well, but it happened bit by bit.
"I finished by reaching a good level, with the possibility of playing a little bit. Unfortunately, we didn't get the chance to win the Premier League".