Aaron Paul Posts Touching Tribute to Late 'Breaking Bad' Costar RJ Mitte

Aaron Paul is mourning the loss of his "Breaking Bad" costar RJ Mitte ... and he says he wishes he had known the late actor was hurting.

Aaron shared a video of RJ talking about the importance of mental health ... he grieved the loss of his friend in the caption.

AP wrote ... "RJ, You have always been an inspiration for so many. I am so sorry you were hurting my friend. I wish we knew."

He continued ... "You constantly had a smile on your face and you took every single challenge that was given to you with such confidence and grace."

Aaron said he had the pleasure of watching RJ grow from a kid into "a beautiful young man."

He went on to say ... "The last time we saw each other I kept calling you Superman. You reminded me of him. You reminded me of him because you always led with kindness and strength. It was blinding."

Aaron admired the way RJ never let the challenges he faced from cerebral palsy stand in his way.

And he continued to mourn his friend, adding ... "I'm so sorry RJ. What an absolute tragic loss. I love you my friend. I loved your mother. Your sister. I loved your laugh and how much fun you had riding the wave we were all blessed to be on with Breaking Bad."

The actor encouraged anyone feeling lost, sad or uncertain to speak up, because "life is so damn special."

He continued ... "There is so much going on in all of our lives but far too much beauty in this world to ignore. We need to focus on that. We must suffocate the noise and focus on each other. Hold each other up."

Aaron concluded ... "Connect with someone you care about and say I love you. RJ, love you my friend. I'm so sorry."

We broke the story, RJ died by suicide Monday in his Texas home. Law enforcement says the investigation is ongoing.