Yes, Ry, you do seem to be deep in the throes of hinge guilt.
Try not to wallow in it, beating your breast. Or not too long, not to the point where you hurt yourself and pervert your true nature.
Mea culpa, mea maxima culpa. See where it was your fault, but remember, Si and Matt are also adults. They also have a role in your current mess, heartbreak, all the resulting bad feelings.
Just as recently as the birth of your son they bonded, cared for you and the premature child, even developed a romance for a while. This has now turned sour, but surely, it was THEIR choices, THEIR decisions, THEIR feelings that were concerned. Maybe you have a stronger personality than either of them, maybe they now felt you pressured them too hard into a romance, but hey! They LET you railroad them.
Now, rejoice that all of you are getting clarity on where you stand. You (singular and plural) are developing new relationships with yourselves, as well as with the 2 others in this V. Lines are being drawn, voices are being heard. This is new, but so crucial for you (and all of us). You're only 33, you've got more than half your life still to live. If you are hardwired to love more than one, this way of being sensitive to the needs and desires of partners will serve you well for decades to come.
I do not recommend retreating permanently, out of guilt, into monogamy, if it goes against your true nature. Even if you live monogamously (for a while... say until your younger child is of school age), if you're anything like me, you will feel attraction to others besides Matt. How will this be dealt with? How will you go forward?
I am not saying deny your guilty feelings. Just encouraging you to know this degree of guilt will not last forever. What comes after the guilt and the apologies?
Love withers under constraint; its very essence is liberty. It is compatible neither with envy, jealousy or fear. It is there most pure, perfect and unlimited when its votaries live in confidence, equality and unreserve. -- Shelley
Mags (poly, F, 61) loving miss pixi (poly, F, 39) since January 2009, living together since 2013
"Master," (mono, 36), miss pixi's Dom for 3 years