This is evidently not a simple question of biology. The erudite and refined users of this website know better than to suggest that one can simply exchange ones lower torso and legs for a graceful fish-tail, grow a pair of gills and some waterproof inner-eyelids. The technology is far beyond us at the moment, especially if you wanted to change every time you changed medium. (We could perhaps remove your legs and add a fish tail, but it probably wouldn't work, and your body would likely reject it)
So what them, is at the heart of this question? It is, I believe, a question of attitude. The mermaid, that mystical figure of grace and wonder, that symbol of a human being somehow altered for the better to its environment, is what you must become. Study how a fish moves in the water, and emulate it, for who knows more about swimming than fish? Move like a fish and learn to swim with the water rather than against it, and you will feel yourself growing more adept in the water. As your confidence grows you will find swimming relaxing, and only then will you become truly graceful.
But is it simply a question of learning to swim? I think not. The question asks how to turn back to a human. And thus remind us that even as we learn the joys of the ocean, we must remember how to walk on land and move amongst other humans.
And so the true nature of the question is revealed. This is not really a question about radical surgery, nor one about learning to swim, but about adapting. The 'sea' and 'land' are not physical region, but circumstances in one's life, and one must learn to adapt to each new circumstance with grace and serenity. To learn to adapt now will not divert calamities from the future, but it will help you deal with them when they arrive, and indeed your calm and inner peace will help others also to cope.