First, congratulations! You're so right, this is a wonderful time for you and being a mom is the hands down best experience! No one can diminish or take that from you.
Second, coming from someone who was in a similar position, I urge you to share with your mom and sister at the very least, maybe your best friend too (I know, from experience that its not easy) I think you'll be very surprised that all they want to do is love you and protect you and support you. Additionally, you'll be flat out exhausted after your son is born and you'll really be thankful that your mom and sister can know now that you'll need the extra support after your baby's arrival.
Lastly, you are not a failure or ignorant or an idiot. You're a loving partner and a responsible mother. You're about to have a baby and you have been trying to provide the best environment for your son. Your boyfriend's addiction is out of your control, its even out of his control. All you can do is focus on yourself and that wonderful life that you're about to welcome into the world. And what a wonderful task to have before you.
Sending you a great big giant hug mama!