Really need advice !!!

I had my third baby nearly 3 weeks ago and I thought I knew what I was doing .. turns out the complete opposite!

1) my newborn was breastfed for 3 days straight.. however we introduced formula around day 4.

2) she's now 4 days away from being 3 weeks old and she still has boob all throughout the day BUT has 2 - 3 bottles of formula during the day. 4oz bottles she usually drinks 3oz but sometimes all, or sometimes just 2oz.

3) she spits up a LOT. She is very greedy bless her little heart even the midwife said she's a piggy ha!!! She never lost her birth weight she gained weight. (She still looks little and a good size)

4) I give her boob, she sometimes comforts herself with it and can fall asleep straight away. (She won't take a dummy). She can be on my boob for 2 minutes or up to 15 minutes. She spits it out when she's done.

5) my milk flow is very fast and I'm wondering if this is why she spits up a lot of milk?? She spits up formula too but sometimes she's very hard to burp.

6) I have to burp her with boob milk because my flow is quick and it unsettles her. Is this normal?

I don't know if she has colic or she just drinks her formula / boob milk too quickly? I really need advice bevause I'm feeling useless and not sure what to do? She's on my boob during the whole night in between her naps but recently she's spitting up and it comes out her nose too it's so scary!!!