What does the church define as mortal sin?
Something that is grave matter and done with full consent. So willfully breaking one of the Ten Commandments would be an example (there are lots of spinoffs related to the commandments so consult the catechism).
For example: to murder is a mortal sin, but Jesus tell us that if we are angry in our hearts towards our brother, then we have murdered him in our hearts. So is anger considered mortal or veneal sin?
Anger can be mortal, venial, or no sin at all. Anger is a valid human emotion. Jesus was angry when he overturned the tables in the temple and we know he never sinned. His anger was a just and righteous anger against mocking God.
I would say anger becomes mortal when it is unjust and harms another, I would confess to a priest most outward displays of anger regardless of where it technically lies.
If one commits a sin such as being angry, and wished to go to confession, what is the state of his soul between the act of committing the sin, and when the individual can go to confession? Can that individual receive communion before going to confession?
Whenever one commits a mortal sin, they should repent as soon as they realize what they’ve done. They should say a heartfelt act of contrition and go to confession at first opportunity. They should abstain from the Eucharist until they reconcile with the church in the confessional.
If you are in doubt, go to confession there is a reason it is weighing on you.
Remember that we are bound to the sacraments but God is not. If you die between your repentance and trip to the confessional, God knows your heart and desire and is just to forgive you.
If you disregard the sacrament or its importance God knows that too, and it may be a sign that you aren’t really repentant.
So remember that this whole system, all these rules…they are about bringing conversion to people’s hearts. They are tools to get us to trust in God and follow him and get molded into Saints. They are avenues to receive his grace.
It is not like God is there with a grade sheet checking boxes. He cares about your heart attitude.