Emma took to Instagram to share her thoughts on her husband's birthday, which she says started out with her crying this morning. The reason ... Emma is obviously dealing with Bruce's recent dementia diagnosis -- and she says it takes a toll on her and their family.

She goes on to say that people are always commending her for being so strong in the face of adversity ... but, as she notes, she doesn't have a choice in the matter -- and simply does what she has to do to keep her household and life in order.

Despite having to put on a brave face, Emma says she experiences times of grief and sadness -- especially with having two young children that she and Bruce are raising -- and as she tearfully explains ... it's difficult.

BTW, Emma says these videos she makes -- giving updates on Bruce's condition and how it's affecting their home life, etc. -- are gut-wrenching to her ... but she does them anyway because she knows how many fans BW has ... who are interested in his well-being/journey.

She says the outpouring of love and support she and Bruce have received means the world to her ... and Emma clearly takes it all to heart. We also know Emma is deeply protective of Bruce these days -- she recently told paps (publicly) to back off and to leave him be.