lilly, you said the client was really pleased, so please don't be too hard on yourself, plus, under any circumstances let the client think that what you have done is sub-standard. You are setting a precedence for your clients to nit pick and come back constantly demanding you do maintenance for free.
We all think this about our work at some time, but THINK It don't VOICE it.
I sometimes think that a massage I did wasn't up to my usual standard, and then look at the client who is lying there in bliss, I quietly ask if everything was alright, and they usually say it was fab! I then quietly retire and think 'phew!'.