負けず嫌いって英語でなんて言うの？

For someone who is naturally competitive, they might describe themselves as a "sore loser." It makes the situation a little lighter, especially if they are particularly competitive.

Be careful if you use it to talk about someone else, though. It could be seen as a little aggressive unless you have a really close relationship.

You can use it like this:
I'm a sore loser.

Or like this:
A: Dude, I hate losing.
B: Haha, I know Jim. You're definitely a sore loser.
A: Got that right.

Any of these expressions is good.
To describe another person: "He's such a bad loser!"
"He hates it when he loses."

I hate losing, I'm a bad loser.
Look at him, he's such a sore loser!

Sore loser = someone who doesn't like to lose.

*I hate losing/I don't accept defeat.
Example Sentences:
I hate gambling because if i lose i get frustrated for days.
He wants to be the winner always even if he loses.

The above phrases mean the same thing - someone who does not like losing and behaves in an angry way after they have lost.