Think back for a moment to when you first met Shane. You thought he was a jerk and stuck up for a reason, he probably was. He probably still is a jerk and stuck up, you are just willing to over look that because he gives you feelz.
When it comes down to it you are the only one that has something you lose here. You are thinking about taking a huge risk to be with a guy that will most likely pump and dump you. Do you think he would stay loyal to Courtney if you showed up tonight and told him to sleep with you now or never? Do you think he would stay loyal to you if you were dating him and some other girl showed up and said now or never? If you leave Dave for him you know that it is only a short fling and you would be left alone at the end. Meanwhile, Courtney and Dave get to comfort each other due to their SO's betrayal.
I will tell you how this ends if you don't cut off all contact with Shane. Shane leaving you for someone else, probably within a year. You alone, lamenting losing the best person you had in your life. Hearing from your parents that Dave and Courtney are happily married, living the life you wanted, and expecting their second child. There is not a happy end to this version of the story for you.
Take an honest look at Shane and realize the horrible drug he would be for you. The high you get from being a crack addict is an amazing rush, at what cost and long term misery for you? You have a choice to make, I hope you make one that will lead to your happiness and not suffering.
If you choose your husband you will need to tell him that you have developed feelings for Shane, you are horrified you have these feelings and you need his help to make sure you cut all ties, forever, with Shane. This means he doesn't get to see or contact them either. And finally I would have a last sit down in a very public place with you, Shane, Dave and Courtney together and tell them that you developed feelings for Shane you won't betray Dave or Courtney and this is your goodbye. You NEVER take another text, call, message or letter from him again.
Good luck, peace and happiness for you in your future.