First of all... you should be totally excited to meet new women before you go out! You are into this game for a reason.. meet women! So you have to be excited about it! Going out is all about fun, once you realise that, it'll make it more easy..
That's some basic stuff.. to manually get into that mood is hard indeed.. but the better you feel, the easier it is to get into the flow..
When you're new to the game, you should get rid of your approach anxiety, just by opening so many girls until you realise that the outcome doesn't matter at all anymore.. You'll get more relaxed among beautiful women.. Being in a relaxed mindstate will make it easier for you to have more fun and talk more openminded.. Get rid of any problems that keep you busy and keep you from feeling bad.. being openminded in combination with a lot of fun, will make it more easy to be playful and attractive. Girls like guys who have a good time.
Combine it with pua material like building attraction and comfort by giving kino, recognize IOI's and it'll make things even more easy.. But it takes time... you'll control it after a while if you go out on the streets or in clubs to approach, learn and test.
you know.. if you're not in the mood, just accept it, don't worry about it.. just hold on to the thought that the outcome of approaching isn't important, it's the lesson you learn out of it.
Yesterday (monday night) i had one of those nights.. i was tired after 2 days of partying and i was at an international student party... It was fun! but i wasn't in the mood, so i just went to some girls and talked to them.. because my bodylanguage was good, they responded positive to everything i said.. but the music was loud and they all had funny accents what made it really hard to understand what they were saying.. it's okay, there will be plenty of next times
..have fun