Peter tells Victoria about leaving Caitlin in the future and that now he can't get back to her. Why can't he time travel back to her?

And if he can't do it now, how do he get to the future in the first place?

Peter is having the same issues with his time travel that Hiro did at first: it is unfamiliar to him and he cannot control it. His moving forward 1 year was a subconscious use of his power. He has no idea how to activate it, much less go to the right time or place with it.

Basically Peter hasn't mastered this ability yet. It took Hiro almost an entire season to master it.

besides, peter's memory has not fully recovered? he seems unsure of some things around him.

i think so..but his memory of his mom and adam seems to be fully recovered

Nah, he remembers everything he even said that to Adam when he got the flashback. He just never used time traveling before.

Agree. he never used the time / space bending in season 1.