Learning to swim. I know that sounds pathetic but my whole childhood I never learnt to swim. I was scared of getting water over my face, scared of leaving the edge, etc etc. Every teacher I'd ever had had tried and failed. Many had got cross and frustrated with me.
Then during my teacher training we were told we'd need to supervise swimming and should be able to swim. We got free entry to the local swimming pool for a term.
I was determined to learn. I went every day. Come rain or snow, I walked to the pool. It was winter and I couldn't afford the hairdryer so some days icicles had formed in my hair by the time I got home. But I was determined not to miss a day.
To start with I couldn't even let go of the edge. After a while a lot of the regulars to the pool, mainly older ladies, started recognising me and cheering me on. Little by little I lost my fear, could let go of the edge, could have water over my face, could trust water, could swim with a float.
The day I first swam a few strokes I had about 20 people cheering for me.
I'll never be the world's best swimmer but I can swim a length or two, swim a bit underwater, and swim on my back. I still rate it as my greatest ever achievement.