I would change the approach, and as @jammykam mentioned, implement it as an action.The sample workflow, already comes with an action to send emails here: /sitecore/system/Workflows/Sample Workflow/Awaiting Approval/Approve with Test/Action
It is using the class Sitecore.Workflows.Simple.EmailAction, Sitecore.Kernel. I would create my own action using that one as an example, or even inheriting from it.
You have to implement the process method, which receives an object of type WorkflowPipelineArgs. In this object, you have the property CommentFields, a string dictionary with the comments of each field.Once you collect those fields, it is easy to include them in the email.
