Grief and sadness vs. self-pity

How do I ensure that grief and sadness don’t spill over into self-pity? I’ve come to believe that it is important and good to allow myself to really experience grief and feel sadness when something bad happens. I think that anger and spite and resentment all cover up sadness and grief, so going right to the sadness is one of the best ways to fight giving into anger and resentment. But at times, it does really feel a little indulgent. What’s the line between grief and self-pity.