First, its perfectly normal.
Second, it takes time for things to become automatic. You can help yourself by making absolutely certain you know all the checks, speeds, calls etc off pat, keep rehearsing them in your mind.
Third, and for what its worth, i did my multi rating with a few thousand hours in hand on single engine aircraft. I was totally comfortable in the circuit, or so i thought. The first couple of sessions in the twin took be right back to my initial ppl. The speeds were higher and it was easy to get behind the aircraft. Inexcusably i hadnt learnt the drills and power settings so not only was i struggling with the extra engine but also a host of new procedures. I put that right and everything else quickly fell into place.
So dont worry it will come be diligent and be patient.