Anyone else have a hard time smiling?

This is me trying

I meant to post this yesterday as a part of No Makeup Monday, but it turned into something else.

So I know a smile can be what ties a look together no matter what the occasion or outfit, but I just can't do it. When I want to smile for a picture or take a selfie my face looks...well, weird. It turns into this tight liped, no teeth showing, looks like I may be plotting something against you in my head type of smile. See?

I'm plotting 

Still plotting on you and kinda judging you t the same time

I know what's in that cookie jar

And it bothers me so much because I want to smile. I've been told I have a very pretty smile when I genuinely do it. I just don't know how to make that smile come at will. Part of me thinks I should stop trying to hard but then I wouldn't know how to even start, hell, I really don't even know how to start trying now. >.> I know some people say practice in the mirror but when I do that I stop because I look at myself and go "...Nope." ;_; There has got to be some way to do this! I just have no clue how. But I will figure it out eventually, this I swear!