I actually thought awhile ago that the reconnecting with Si probably wouldn't work (due to how strong Matt feels about it), so I'm not completely surprised about your separation from her. Perhaps what cemented that course of action was the time when you spent all day with Si and Matt got upset about it? or even the time when you and Matt had to curtail the public displays of affection in front of Si. Things like that will give you pause and make you think, "This isn't going to work."
Love means never having to say, "Put down that meat cleaver!"