HELP?! Bad news today

I'm 18 and I'm 21 weeks 3 days pregnant with my second child. I had alot of preterm labor issues with my first because my cervix vanished at 23 weeks so iv been considered high risk from the beginning with this one. Therefore I see a high risk dr every two weeks for an ultrasound the moniter my cervix and the baby.

Today at my high risk check up they found a clot where the umbilical cord and the placenta connect. Iv not bled or anything and so far baby seems fine (she's always been on the small side) they told me to take a baby aspirin everyday to hopefully thin it out, but they are worried if it doesn't go away or gets bigger it could stop the flow through the cord and stunt or stop her growth or even kill her. So they are going to moniter it and her growth extra close. Anyways my question is I'M FREAKING OUT. has anyone been through this? I have so many questions and no answers. How could it have formed? Why? What's the possibility of it going away on its own? How soon could that happen? Will they take her early if it doesnt? (Later down the road of course) what problems besides stunting or stopping her growth could it cause? If it does mess with her growth and getting nutrients and blood flow to her what's the long term effects of that? Won't the aspirin that's ment to thin my blood cause problems with birth bleeding when that time comes? Any stories or tips are helpful.. sorry it's so long..