Do you mean you lack the confidence to tickle someone full stop.... or just that when the chance arises you are so conscious of tickling being your fetish that it's making you nervous and stopping you from actually tickling them.. eg someone tickles you but you're too nervous to tickle back? (Whereas in the same situation someone else would take the chance without a second thought).... I wasn't sure...
Either way, I think it's more to do with the ability to rationalise your thoughts and recognise when it's naturally appropriate to tickle someone, which will depend on your relationship with them and the scenario you're in at the time.
It's not as simple as just having the confidence to tickle someone, but more having the confidence in yourself to get used to when it's acceptable to do so and then taking the chance - they may indeed not be ticklish, but this won't matter if you've assessed the situation correctly, it will just be mildly frustrating and embarrassing. You also need to be confident that you have the clarity of mind to recognise when someone wants you to stop tickling - when the tone in the voice changes, the facial expression changes etc - this will also help as you'll be more confident that you won't upset them.
Your confidence will grow with each time you get it right.
Drunken, happy social occasions are usually a good place to start testing the waters with a few tickles.
A little reference to my own confidence in the early days of my ticklish fetish..... I recall a time in school where a guy was getting the crap tickled out of him and I was mesmerised... all of a sudden his shoe fell off in the struggle right in front of me and one of the protagonists shouted 'get his foot' - in this situation it would have been entirely acceptable for me to tickle him seeing as everyone else was and it all seemed to be in good spirits, but I froze and did not tickle him, something I regretted ever since! I guess I wasn't confident in that particular moment as I wasn't at all prepared for it... at that time I was so blown away by the tickling going on in front of me I had not even considered whether I'd have the confidence to join in and I was very much aware that the tickling was probably having a much more different affect than it was on the others!