Well, here is the issue. Introducing a monogamous person to polyamory by saying "I've been lying to you for years" is really not easy to do nicely. You want her to see the goodness of polyamory... But a huge part of it is the honesty; and you have shown her that both you and her boyfriend cannot be honest.
Getting her to trust that you will be honest in the future is just as big an obstacle as getting her to see poly as a good idea. If someone hasn't chosen polyamory on their own... It's a shot in the dark if they are willing to change their lifestyle.
If there is one thing I've learned.. It is never lie to someone even if it is just to protect their feelings. What you have done with her boyfriend can only be described as cheating. Getting that out in the open is necessary, even if that means she leaves him. It might be painful, but it is respectful of her.
That is the best I can say for this type of situation.