Stus advice is bang on, just work within your comfort zone.
If the tune isnt in yet but you're guna miss your cue, miss your cue, don't fucking clang for the sake of mixing it in a little quicker, just wait for the next phrase.
When shit gets tricky, try and keep your cool and imagine you are at home.
I remember my first vinyl gig and was SHITTING myself, as soon as I started playing my mind just imeadiately switched off to the outside world and just felt like i was at home - Was a really weird feeling actually which i remember well.. That was fucking 7 years ago now! Urgh
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Also check the beat matching in your headphones before bringing the fader up, using monitors on a big rig can be mega different to low level mixing at home.
This is assuming you've never played out at all of course.