Hey guys,
when I'm with a girl I really like i tend to 'lose my game' and act all wrong.
do you have any tip or anything that I can say to myself that will prevent this from happening in the future?
thanks!

I'm in the exact same position. I cannot give you the real answers you are looking for but I can say my experience...
Time after time I would talk to sets and make mistakes without realizing at the time, then look back and realize what i did wrong. I can get past the opener without a problem, then thats when i start to 'lose my game'
I had the same issue with working on my body posture, but one night i went out and kept telling myself 'MUST KEEP POSTURE' and i put that in front of picking up girls that night and put it as my goal. I kept doing this until you know what? I broke the bad habit and now my body posture is as good as ever.
Today I realized 'what if I could do the same thing but for in game?' So next time im in set, i'm constantly telling myself 'keep your game' and start thinking before i say or do something with the girl. Eventually it should break the bad habit the same way it broke my body posture and become natural right?
Maybe this could work for you too? As long as you know all the routines and tricks and how to apply them it should work over time.
Also another problem i had when i started game was I was caught up in so much stuff at once instead of working at one thing at a time, which i think may have had an effect on this as i would get confused on what to say or do so would start acting normal/boring that would fuck up my game. If this is you, then you need to work on it.
Like i said im not sure if this will 100% work for you but its the only advice i can give. Hope this helped you, good luck.