The 31-year-old actor - who portrayed the titular character for 151 episodes in the sitcom - has memory loss after suffering nine concussions and several "mini-strokes", and the 'Agent Cody Banks' star feels "sad" about not being able to recall certain life situations.
Speaking in a 'Dancing with the Stars' rehearsal video on the ABC show - on which he is currently competing - he said: "It makes me a little sad. Things pop back into my mind [that] I should have remembered."
Despite his memory loss, Frankie is "happy" with the acting career he's had, which also saw him appear in 'Arrested Development', 'Dr Dolittle 2', and 'Lizzie McGuire'.
He said: "Truth is, I don't remember much [of my time on 'Malcolm']. It almost feels like it wasn't me.
"My most memorable year is 2017, because I learned to live in the present.
"I'm happy with my acting career, I'm happy with the decision I made to drive race cars and to focus on music. Even if I don't remember it all, I'm happy."
The actor-turned-race car driver and his girlfriend Paige write down what they do together to ensure he doesn't forget.
He added: "Like, I should have remembered going to Australia. That's something that people remember."
Bryan Cranston, who played Frankie's on-screen father Hal in 'Malcolm in the Middle', has been amazed by his former co-star's career path and often reminds him of certain episodes of the sitcom.
Speaking on the clip, he added: "Frankie was a TV star, then he became a race car driver, and then a drummer in a rock band. I said to him, 'What are you going to be next, an astronaut?'
"They are still [his] experiences. My job is to tell him to remember what happened on 'Malcolm in the Middle'. What a life."
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