I think it's okay to bend the truth a little sometimes especially if it's related to ones mental health issues. Society just can't handle the straight up honesty regarding mental health a lot of the time, then start leaping to all kinds of immature, ill-informed assumptions about the person - and that sucks - which I know from my own experience. If I want to avoid or cancel something on mental health grounds I basically just say I feel sick [which to me is actually very true, ha]. But if I have a trusting relationship with the person I'll give more detail. But I hear what you're saying as well - I don't like lying either. Just don't tell whoppers which I doubt you would ever do.