I acted like a jealous bastard. How to fix it?

So I went out with the gf & my best friend for dinner.

Both of them seemed to hit it off a lot. I got jealous and acted cold and irrationally to both of them. She says I'm not the calm man she first fell in love with.

Until now, I've never done anything that would indicate that she could affect me so greatly (which is what I attribute some of her interest in me to).

So how can I change things and put our relationship back on the right track where I am seen in a more positive light and seen to be as calm and unaffected as I once was (if that makes sense)?

You can't change what has been done. Don't dwell on it. If you do it will effect your inner game. Just focus on not doing it again.