You HAVE to trust the instruments, and sometimes it ain't easy!
Do this is cloud - total visual sensory deprivation from the outside world. No cues of up / down or straight / level...
Ears and stomach say: "We are turning left, make a correction to maintain heading"
Eyes: "No, we are straight, and level"
E+S: "No, can't be, I can feel it, put some right stick in"
Eyes: "No, look, the heading bug isn't moving"
Cue battle of the senses, until the newbie VFR PPL makes the common sense judgement and turns the aircraft to the right.
E+S: "There - thats better we are flying straight and level now"
Eyes: "Oh no we're not, look we are turning"
E+S: "We can't be turning, this 'feels' nice and straight, I'm not leaning due to turning forces anymore"
Eyes: "Look, there, look, we *ARE* turning, the AI says so, the HSI says so, the compass says so"
E+S: "Mmm.. you may have a point there..."
Another battle of the senses, IMC student takes over and turns the aircraft to the left to align the instruments. Maintain the instrument scan. Everything is OK again.
E+S: "Woah! Don't like this, we are turning again, look, I'm even leaning over to counter the forces"
Eyes: "Aw, just shut up and look at the instruments"
And so the battle continues... all the while the instructor is probably having a quiet chuckle.
But it does get tiresome and confusing. And it is so easy to get out of line if you don't maintain that instrument scan all the time! Even short actions such as radio changes and VOR idents etc. can throw everything off course.