Open up when:
You are clear on your feelings and wants and how to express them calmly with love
You trust the other person - they demonstrate integrity and emotional maturity
The other person is calm and ready to listen
You don't think you can find a way to be okay with the current dynamic or situation - something on the outside has to change
Sit with the emotions when:
You are unsure of what you want to say (if you continue to feel confused over an extended period of time it may be worth opening up to see if the other person's perspective brings clarity. Saying you are confused and explaining multiple perspectives you have been switching between is also having something worth saying)
You feel too much anger to express without blaming
You doubt they would be able to understand /empathise (you might still need to find out if they can but restoring inner calm first might make the conversation more productive)
The other person is too angry to talk/ needing space
You realise that what has to change is your perspective, not their behaviour (you might share this realisation later)
The main consideration: Am I able to act from a space of love rather than fear? Secondary consideration: Are they able to act from a space of love rather than fear?
You may be able to increase the likelihood that your person will be able to respond with love by giving them some time and space and waiting until a calm moment to present your perspective. Sometimes what you have to say will ask to be said despite not knowing how it will be received, due to a calm, intuitive knowing that it's in everyone's best interest or because their response will inform what action you take going forward e.g. ending the relationship, rearranging a plan they were an integral part of.
You can ask yourself 'how urgent is delivering my thoughts?' - sometimes we might express sooner because we desire to release the negative energy we are holding onto - but this can lead to aiming your pain At the other person, which is more likely to upset or anger them and cause an emotional fallout to repair on top of resolving the original challenge.
Healing is in our hands. We can release negative energy through our own internal practices - anything from journalling to breathwork to cardio to punching a pillow.
Sometimes we become 'I-centred' and push for a conversation another person isn't ready for. Often, being in surrender and letting them move through their process shortens the time it takes for them to come back around and be ready to listen with presence. And there is also the option when a person has been avoiding us for some time that we might express our intentions or a boundary:
"I'm unsure where we stand - if you're unable to communicate with me I will assume ___/ I will have to ____ for my own wellbeing"
When you are connected to your intuitive voice, which is free from fear, you will know what to do.