Should I tell my mother what I did in the past, even though I have repented, and I fear her thinking ill of me?

You did well to stop chatting with this man online, as it is not permissible for a woman to establish any relationship with a man who is a stranger to her, in order to block pretexts and prevent temptation. Your lying to your mother is a great evil and an act of disobedience (to parents), and for that, sincere repentance is required, which includes abandoning the sin, regretting it, and resolving not to return to it in the future. You must also frequently supplicate for your mother and be truthful with her in conversation.