Dear Mr. Anonymous,
Both of You Are Sensitive and Emotional
You make a very strong case for taking some more time here. It sounds like you and Shannon are both sensitive and emotional people, and those qualities can be good and bad in a relationship.
Her Reaction Seems A Bit Over the Top
Your bond must have been a very strong one indeed for you to have seriously considered marriage. If so, her reaction seems a bit over the top to me. If NOT, all the more reason for you to take some more time to figure out if this is the right woman for you.
You Should Be Your Top Priority
And please, don't be swayed by her entreaties to "not to let her live with a mistake the rest of her life." That is HER problem you need to think and act according to what is best for YOU. I don't mean you should disregard her feelings, but your happiness and emotional well-being are top priority here.
Start Over Without Being Serious
I wouldn't jump right back into a serious romantic relationship here. If you think it's worth it, try "starting over" as it were, talking a few times a week, maybe going out occasionally, see how it feels.
Don't Tolerate Her Games
Don't hold the past over her head, but make it clear that you won't tolerate the games she played in the past. If you feel toyed with or mistreated, bolt and find your happiness elsewhere.