You should be working with a teacher is you are new…especially with shoulderstand and Plow(Sarvangasana and Halasana).
And what is Paschchimatanasana? Paschimottinasana is a seated forward bend…so why are you lying down?
Why are you launching yourself into an asana???
There should never be any uncontrolled type movements…this could led to injuries…you should be moving/easing into all asanas in a safe controlled way…no launching…
Who taught you this sequence?
Why did you choose this sequence?
The asanas listed look like strong forward bends alternating with back bends…which is not a good idea.
Are you doing any other asanas?
Do you have any health issues?
Are you going to a teacher?
Breathing through the nose is better for you than mouth breathing.
Mouth breathing can make your mouth and throat very dry.
Mouth breathing can also led to other problems…and other problems such as panic attacks , stress or cardio or respiratory diseases can led to mouth breathing.