There is no easy way around this and there is no shortcut. You have no choice if you want to be honest and intimate with your boyfriend, but to tell him about your past. If you don’t, he’s going to find out eventually and feel you lied to him, or he may never find out, but then YOU will know you purposefully withheld and lied to him. So do the right thing and tell him the truth.
Preference the confession with your feelings of fear and your apology for not having had the courage to have told him sooner, and then just blurt it out and let him react. You don’t know what his reaction will be, but until you give him the opportunity to react, and to have feelings and express them to you, you’ll never know.
So take a deep breath, blow it out, and then tell him. You’ll feel better for having finally told the truth, and this will help you with other difficult situations that may come your way in which honesty is called for.