ノミの心臓の私には、ホラー映画は無理だ。
With my cowardly nature, horror movies are impossible for me.
小心 means timid or faint-hearted, often used in compound words like 小心者 (coward). ノミの心臓 is more colloquial and vivid.
Common katakana-kanji mixed spelling for the idiomatic expression.
Kanji spelling using 蚤 for flea; less common but still encountered.
Literally 'a flea's heart', an idiomatic expression comparing a person's courage to the tiny heart of a flea, implying extreme timidity.