You can't just change the grip and keep all other things (shoulders, legs, toss) the same. I would bet that you previously served by pointing your open chest straight at the target, with a ball toss similarly positioned straight in front of your face/chest. With the continental grip, you should be rotating your whole body 90 degrees, so your shoulders line up with the intended target (you are facing the side of the court). Likewise, your toss should be in front of your backhand-side shoulder.
I find it way easier if you, instead of trying hit the back of the ball for pace, focus on brushing the back of the ball for lots of side spin (swinging parallel to the baseline). ***Stay sideways at first!*** Your muscle memory will lead you to turn back to your old motion as soon as you toss, so minimize your shoulder rotation at first.