"It has been the honor of my lifetime to serve as your First Lady. I want to thank the millions of Americans who supported my husband and me over the past 4 years and shown the incredible impact of the American spirit. I am grateful to you all for letting me serve you on platforms which are dear to me."
If we are to become nit-picky, it should state "I want to thank the millions of Americans who supported my husband and I over the past 4 years and have shown the incredible impact..."
"It has been the honor of my lifetime to serve as your First Lady. I want to thank the millions of Americans who supported my husband and me over the past 4 years and shown the incredible impact of the American spirit. I am grateful to you all for letting me serve you on platforms which are dear to me."
Incorrect tense. Should be "showed".
Search "shown" on .pub. You're welcome.
If we are to become nit-picky, it should state "I want to thank the millions of Americans who supported my husband and I over the past 4 years and have shown the incredible impact..."
Q doesn't expect us to rewrite sentences. Typos themselves give clues.