Are you using regression of ANOVA to do this model?

I am not sure from your comments if the measure of pessimism is interval, ordinal, or nominal. And the same for your dependent variable (I am not sure exactly what your dependent variable is). Depending on which they are you can run different forms of regression with the predictor if they are optimistic or pessimistic against symptoms. You would code your predictor as either a dummy or interval variable depending on what form the data is in (that is if its interval or not).

You need to provide more details on how you measure your variables. Scale can mean many forms of data some nominal, some ordinal (this is most common) and some interval.