So its been over a year since I was in a 'relationship' and I've just gotten into something more serious with a girl I really like.

But something is concerning me.

Because I've been single for so long (I'm 19 and a year is a long time for me) I've gotten used to flirting with girls and being that sexy flirty guy... so I know how to act around girls in that way... but now that we've started something more serious things are getting deeper... and I don't know if I'm being too full on or not... I don't particularly have any commitment issues... I'm certainly not thinking of other girls at all so that is no problem

I know most people will be saying that its only just started and I need to give it time to develop to know how to react to things and it will become natural, and I realize that.
But I just want to know why its now difficult for me to be the "boyfriend me" instead of the "flirtatious me" and why these boundaries exist.

People are always saying to get a girl interested to play the mysterious/flirtatious card (generally speaking). But now that its more serious.. I don't know how "un-mysterious" I should be.

My last relationship ended a disaster after a year together... and I was much younger and more immature then... so the way I acted in that relationship would naturally be different now, right?

Does someone know what on earth I am going on about? Any comments will help