My name is Thomas. People often call me Tom, as that is a standard nickname for Thomas. However, I have always gone by my full name.
I don't identify with the name Tom. When it is used, I have to consciously remind myself that this name is referring to me and not someone else. It's not something that upsets me, but it's distracting. I know they are acting in good faith. It comes across as someone trying to sound friendly, but it actually emphasizes how little they know me.
How can I gently indicate to people that I prefer my full name? I don't want to offend them (e.g. by seeming too formal) and it's not a big enough deal to me to really worry about.
This issue arises in many contexts -- people I meet at a social event, my accountant, my manager at work, in conversations, in emails, etc.
(On the opposite side, there are questions on this site for someone who prefers a nickname: How do I get someone to call me by the name I prefer? details another party refusing to use a preferred nickname even after the topic has been explicitly discussed. And How to introduce your nickname to new people? applies to initial introductions only.)