Need advice about smiling

So sometimes during conversations, I start smiling but after a bit, it reaches a point where I want to stop smiling but am unable to do so in a manner that looks natural. My cheeks/lips start feeling shaky and become difficult to control.

So I either have to keep smiling very uncomfortably or stop smiling abruptly. Only thing I can do is look away before I stop smiling (if possible).

Of course, I only start smiling when something genuinely makes me smile (I can’t help smiling when this happens) but if I stop smiling abruptly, it might come across as a fake smile and might even seem rude.

Also, this doesn’t happen when I am interacting with people I am used to interacting with.

So yeah, I need some advice on this. Thanks in advance.