When you use the opposite word by mistake

Solution 1:

Not a single word, but Wikipedia has an article on speech errors and one of the examples for substitution errors is just what you describe

Substitution errors, for instance, reveal parts of the organization and structure of the mental lexicon.

Target: My thesis is too long.

Error: My thesis is too short.

More details:

In case of substitution errors both segments mostly belong to the same category, which means for example that a noun is substituted for a noun. Lexical selection errors are based on semantic relations such as synonymy, antonymy or membership of the same lexical field.

Which maybe leaves you with antonymy substitution error?