The only person who can begin to understand it is you. For someone else to truly grasp what you're going through, they would have to step inside your mind and your body, feel every memory, every scar, every echo of what you've lived through. And even then, understanding your pain is not a moment. It's a lifelong journey, a horrible lifelong journey.
I think about this often. You should read Wittgenstein, he often talks about the limits of language and how every word elicits different emotions and meanings for every individual. That we all live in our own internal worlds with our own relative feelings and language is our shitty attempt at bridging those worlds. I can say I am sad, as can you, but what sad is to us is defined by the lives we've lived and the thoughts we have, so even if we believe we are relating to one another through our shared sadness, we couldn't be further apart.
It tears me up inside sometimes, as it makes me believe that genuine connection with another human being is not possible. Maybe the key is not to understand the other in their entirety, but for both parties to acknowledge that the other suffers. It's to know that both your reasons for suffering are completely individual and unknowable, but these circumstances have warranted you to decide that yes, you do suffer, and you can relate to and understand the fact you both suffer even if you cannot know how they suffer.
I think this is how true empathy words. A lot of people say that empathy is putting yourself in the other person's shoes, but I do not believe this. Think of a child who's favourite toy has broken, and they begin to cry. If we were to put ourselves in their shoes, we'd think "It's just a toy, I can just get another one, or I'll get over it eventually.", so we end up thinking the child is unreasonable. Instead, I think the key is to imagine a time when you lost a person or item that was valuable to you. Let's say your best friend dies, it gave you an overwhelming sense of loss, the same feeling of loss that child is feeling. The key to empathy is to relate a feeling you experienced to what they are experiencing, rather than trying to feel exactly what they feel. It's all relative.