I agree with your mom and your therapist. But perhaps for a different reason.
If you want the relationship with your boyfriend to continue, you should probably let this go. If you do tell him, he will probably break up with you. If you’d rather be super honest and know that in doing so, this late in the game, your confession will end the relationship, then proceed, but be forewarned. If you can see this as a “little white lie” that’s part of your past, then let it go and move forward.
I hope that helps. 🙂