I'm not a Psychology student but I think if you really put your mind to something you stand a good chance.
I did Bio at AS and had a D and found out our teacher didn't teach us the whole course (i had 100% attendance so missed lessons didn't not contribute). I asked my teacher on my Year 13 parents evening if it was possible to go from a D to a B and he was really horrible about it. He quoted some stat saying that "0.02% of students improve by 2 grades at A2". I told him to get lost and that I was going to be one of the 0.02%. Apparently that stat is rubbish.
What I'm trying to say is, if you really want to get that grade and you aren't afraid to seek the right support you'll do it! And even if you don't get the A, getting a B at A2 is such a big achievement from a C at AS that your uni may take you anyway! That's what happened to me.