Restudy the illusions associated with instrument flying. Remember them so you can be ready prior to experiencing them.
If you still are not comfortable, grab an instructor, tell him/her not to say or do anything unless you are going to die. Now go fly in the clouds: seek out low IFR. Practice, practice, and more practice.
Over time, your disorientation experiences with shorten as you gain trust in the instruments and your skills.
I still occasionally get very short disorientation experiences. When they happen, I re-intensify my scan, mentally hug the attitude indicator like a teddy bear, and the felling goes away in a second or two.
Don't worry. Nearly all new IFR pilots feel like you (IMHO the ones who don't are more likely to ball one up).