The poet is talking about his fear of death. Ceasing to be is a common way to describe dying. This means he's scared of dying before he can finish all he has set out to do. Perhaps he is scared he hasn't done enough in life to deserve the release of death yet.
And think that I may never
And think that I may never live to trace
Here the poet is talking about how he has never gotten the chance at romance in his life. Which he represents with "The cloudy symbol of high romance" Maybe the reason he is afraid of dying is he doesn't want to die alone. Maybe he wouldn't be so scared of dying if he knew he had somebody who cared of them.
The tone of the sonnet gives a picture that he isn't uncertain about fate, but is rather fully aware of what comes next in his phase of life. He doesn't ask questions he simply answers them to the best of his ability. This gives the tone of a grim understanding. He is saved from the fear of the unknown. In exchange however, he is required to be fully aware of his mortality.