Originally Posted by polycouple
There is no history between Tom and I, but there is history with Tom. He has a past of infidelity with his past long term relationships.
Yup, there you have it. Imagine Tom and Sara would have a history of communication where Tom would be the assertive, confident decision-maker and Sara would have a pattern where she would be too timid to voice objections out loud and thus would go along with things she was uncomfortable with only to please Tom, while harboring resentment and lashing out later at most unexpected times. So Tom and you would make holiday plans for the three of you, with Sara feeling she must oblige or seek angering/disappointing the two of you. Then, week before the said holiday, she would break down in tears and start shouting that the two of you are leaving her out and making all the decisions without a care for how she feels.
In this scenario, the pattern was there before you came along, and you did nothing to bring it about - your presence made the problem more acute, but didn't create it. The same goes for Sara's insecurities about Tom's past behavior. You are not the cause of this - nobody really is the problem here, so I think you really can work through this.