Final answer:
Othello acknowledges his insecurities regarding race and age that may cause Desdemona to be unfaithful.
Step-by-step explanation:
In the soliloquy starting at line 299 with the phrase 'This fellow's of exceeding honesty,' Othello acknowledges his insecurities that may cause Desdemona to be unfaithful. One reason is his race and the societal prejudices against him as a black man. Othello worries that Desdemona may be attracted to someone of her own race and social standing. Additionally, he feels insecure about his age and lack of experience, fearing that Desdemona may prefer a younger man with more life experience.