When I speak to my potential future wife, should I tell her if I have had unlawful relations with a girl in the past which I have repented for. I’m afraid of not telling her and if she finds out later and asks for a divorce based on that.

You should not tell her about that. First because we should not disclose our sins to others. Second, it could push her away. No need to bring it up. If she happens to discover about it later, you can effectively explain to her that you repented and that Islam does not permit us to reveal our sins to others, and that’s why you didn’t tell her. Have Tawakkul on Allah and hopefully it’ll work out.