Don't be scared, it's really important to get to grips with different lengths.
I'm not sure which sizes you have or if you use loose or strips of lashes?
but, as a basis for your model tomorrow to get use to the lengths... Try using 9mm,11mm &13mm (usually the most common sizes)
When attaching them to your hand put the 9 closest to your fingers, so at the top of the back if your hand.
Then 11mm in the middle,
And 13mm at the bottom. If you always keep them in size order from top to bottom you'll never get muddled with what size you use.
Next, try using 9mm on the inner 3rd of the eye, 11mm on the middle, then 13mm on the outer.
Work from side to side, so one 9mm lash on left. One 9mm on right, one 11mm on left, one 11 on right, one 13mm on left, one 13mm on right then back to 9mm on left, 9mm on right...and so on...
This way you always know how far you've got and both eyes will be even.
Now this arrangement of sizes does not work for everyone and 13mm can be too long for some people. And you will find that on the very outer lashes you might want you use a size smaller to stop them looking spidery, being too heavy, pointing in wrong direction or hanging down... But it is a really good arrangement for while you are practising and getting confident.
I use around 6 different sizes/thickness or curl per client sometimes but you will learn differences as you progress and learn which works best and what suits each client.
But for now just stick to getting confident. Learning to use different lengths. Getting lashes on those outer corners.
It will get easier
xx