It is good that you are proceeding with caution in this situation for several reasons, the most important one being that she is your MIL and keeping things amiable is important. Sensing that she has a strong negative attitude about mental health issues, maybe keeping your own issues to yourself would be best. I don't think that is necessarily 'hiding' something ... it is just choosing wisely who you share your stuff with and that would apply with any personal issue, not just mental health.

If you choose to handle things differently with your MIL, believe me, you will get another opportunity. It sounds like she doesn't like to keep her opinions to herself.