When we paraphrase something, we are essentially extracting the meaning of the text and rendering it into words more readily understandable. Paraphrasing Shakespeare can be useful because it forces us to parse the language carefully and make sure we understand what's being said.
This speech of Macbeth's is spoken as an aside, to himself, but also to the audience, of course. Effectively, Macbeth is saying something along these lines:
Two of the things the witches have told me have turned out to be true. This suggests that these first two good things are only prologues, or lead-ins, to the best part—me becoming king ("the imperial theme").
This doesn't seem like it can be a bad thing, but it also doesn't seem like it can be good. If it's a bad thing, why have I already had some good experiences as the witches predicted? I'm thane of Cawdor.
However, if it's a good thing, why do I feel so strange inside—why do I keep thinking about this "horrid image" which makes my heart pound and which seems unnatural?
The things I'm afraid of at the moment, the real fears, are not as frightening as the things I find myself thinking. At the moment, it's only a thought (that I could murder the king) but it makes me feel very unnerved and shaken. I'm really worried about this even though it isn't a reality yet.