For me it’s public speaking. Three years ago I could hardly bring myself to talk to any strangers. Every time I tried it appeared to be a disaster.
Now I’ve reached the level where I can talk on a professional level (e.g. sales people, teachers and customers), but I still have trouble with social conversations.
Still, to learn to talk normally to at least some people was pretty difficult. I must have made a fool out of myself countless times, but I think it was all well worth it.
What was difficult for you?