Fanfictions depict the 10th Doctor's emotional journey during regeneration in various ways. Some show him being in denial at first, refusing to accept that he has to regenerate. He could be bargaining with the universe, trying to find a way out. As the regeneration approaches, there's a sense of desperation. He might be trying to hold on to his current self as tightly as possible. And then, when it finally happens, there's a mix of fear and hope. Fear of the unknown and hope that the new Doctor will be able to carry on his legacy. These emotional arcs are what make the fanfictions so engaging.
In many fanfictions, the 10th Doctor's emotional journey during regeneration is complex. He not only has to deal with the physical pain of the process but also the emotional turmoil. He may be afraid of what the new version of himself will be like. There are also memories of his past regenerations flooding back, adding to his distress. And his relationships with his companions play a huge role. He might worry about how they'll react to his new form, and whether they'll still be by his side. All these emotions are often explored in great detail in fanfictions.