After a while you begin to get used to doing mantras and it no longer becomes something you stress or focus on - simply you something you do and believe in. Personally mantras work brilliantly for me, and I've experimented until I've found mantras which work really well for me.

Experiment with various mantras, and over time you will begin to be able to do them a lot, without the stress, and fall asleep with MILD on your mind