When do you pat yourself on your back? If you can reach it, of course.

When I'm having a conversation with someone and don't say something ignorant. 

I did that once
...it was a special occasion. haha

Can still reach all parts of my back with my hands.
Might sometimes slap my back to get rid of a stinging fly.

For me there's nothing intrinsically pleasant about a pat on the back.

In fact, I'm a bit amazed that as a gesture or an idiom in our language it's meant to indicate a reward.
People even pat their dogs and horses. They don't massage or stroke, they give them a series of small strikes with the flat of the hand. You only have to watch the reaction to see that the animal tolerates but does not enjoy it. What on earth is so wrong with humans that they don't have the empathy to see this?

Rather, if I'm feeling pleased about having accomplished a challenging project, I'll give myself a treat, usually some time to relax and do something enjoyable.
If it's something really big I'll have a little celebration, mabe a very nice meal with someone special.

What's your equivalent of a pat-on-the-back, Ele?