If channeled properly, they can be constructive i.e. ones that empower the self; if not, they can be destructive i.e. ones that bring the self down or weaken it in some way or the other.
They possess immense potential, both positive and negative, if we want we can use them to liberate us or they can even limit us. We can judge, criticize and threaten ourselves or praise and encourage the self in a positive way.
It's important to create thoughts of acceptance, respect and approval for ourselves, even if situations don't go the way we desire or expect. Such positive conversations help rise above the dependence on what how people see us and what they think and talk about us, making the self extremely powerful.
Positive self conversations are not fantasies or disconnecting from reality and a few minutes given to the activity do a lot of benefit and no harm at all. They can be done by detaching oneself from actions when we have an emerged conversation and our complete focus is on the conversation and we do nothing else at that time or whilst being involved in actions. Some actions like going on a drive, cooking, having a meal or simply taking a walk or going on a jog allow self-conversations to take place alongside.
At these times we give less mental energy or attention to the conversations i.e. we can have a merged conversation so that the respective action is not affected adversely in any way. We can choose how much mental energy to give to the activity, depending on the karma alongside.