Illinois has not enforced Breach of Promise for many years. You knew he was married when you began the relationship. Don't talk to him until he can show you a Petition for Dissolution of Marriage is filed. Don't be attentive to him until he can show you a Judgment of Dissoltion of Marriage. If he won't take these actions, he is not committed to you. As I am often in Court call my assistant Dan London to set up a private telephone consultation. At the consultation we can discuss the confidential details we need to craft a strategic plan. These details should not be broadcast over the Internet.
This is a general answer and does not address the specifics of your individual case. To give the specific answer you need our firm needs you to come in for a consultation.
No, you cannot sue your boyfriend. Apparently he is now your former boyfriend. Well, actually, you can sue anyone for anything but the question is whether you can win, and the answer is you have no cause of action and cannot win.
You have no legal claim. In fact, it can be argued that you "assumed the risk" of this eventually happening by dating, etc., this man for six years based upon a tenuous representation, at best. As bad as this must hurt, emotionally, etc., this is the best thing that happened to you. With a little time, you can leave him behind and move forward to finding someone worthy of your affections.
Oldest story in the cheating husband handbook-- the 'other woman' never hears the actual truth.
READ THIS BEFORE CALLING OR EMAILING ME: I am licensed to practice before the state and federal courts in Virginia. We have not established an attorney-client relationship unless we have a signed representation agreement and you have paid me. I am providing educational instruction only--not legal advice. You should speak with an attorney to whom you have provided all the facts, before you take steps that may impact your legal rights. I am not obligated to answer subsequent emails or phone calls unless you have hired me. I wish you the best of luck with your situation.