what are the short messages about the story my bondage my freedom

Do you mean like themes of the story. Themes are messages or ideas that the author wishes to get across to the reader. I would consider ideas like ignorance being the tool which keeps slaves in their place. THe slaveholders knew they must keep their slaves ignorant because learning empowered them. Douglass was able to overcome many obstacles through education and knowledge. Both education and knowledge emancipated Douglass from his on darkness, the darkness of ignorance and self-loathing. Once he had knowledge (like ship building) and education (ability to read) he could no longer be a slave.