「本音を言う」means "To speak one's true feelings"; however, when used in the form of「本音を言うと、…」or「本音を言えば、…」at the start of the sentence like this, it often means "To tell you the truth, (...)" or "Honestly( though), (...)" etc. The「…ほうが…」on the other hand denotes comparison to something else, and the「ホッとできる」is just the potential form of「ほっとする」, i.e. "to feel relieved". It's often written with katakana like this, but the meaning is the same as with the hiragana form.
As a whole, the sentence translates to something to the effect of "To be honest, I('m able to) feel more at ease when I'm alone in my own room", although the details would vary slightly depending on the context.