Have no trouble at all striking up a conversation with most girls I find attractive, or anyone else for the matter, BUT -

when the moment comes about to ask her out/her number, for some reason, I tend to automatically repress the question, wish her a nice day/evening, or worse, say bye awkwardly, and continue with my day. Feeling like I'm losing some really good opportunities because of this.

Certain thoughts crop up blocking me from moving forward.

How can I fix this?