I don't know how to ask this, but I know it is an issue with me I'd like to control more. How can I stop long-term grief? Is it ever possible to do so?

In my own personal experience with trauma and grief, I believe "stopping" it is all the wrong of a term to use. Many events, traumas, and experiences can lead to long-term grief, and this grief is brought about for a reason. It's important to validate your own feelings and emotions, as it's necessary in order to overcome any troubling thoughts, feelings, or emotions. I'd suggest looking deeper into what is causing you this grief, if you feel you already have a good grasp on it, perhaps try viewing it from another person's point of view. In all hopes, this will let you view it as less of an "issue" with yourself, and more so a sign of humanity. That step may take longer for some than others, and it may be something you need to remind yourself of on multiple occasions. Regardless, it's necessary in order to begin coming to peace with whatever the cause of the grief is. Eventually, there's a time where one realizes the futility of allowing the past to influence or have control over their future.