It appears your epub has 'broken paragraphs'. I suspect your source mobi has the same problem, garbage in - garbage out. You can check whether this is true by opening your epub in the calibre Editor to see if the paragraphs are broken.
Was your mobi originally created by converting pdf-->mobi? Broken paragraphs are typical of a bad conversion from a pdf source. PDF conversions are notoriously problematic. However, if you find the original pdf source you can try converting to epub in calibre and experimenting with the 'line unwrap factor' on the
Convert - Heuristic Processing page.
Alternatively, if you don't have the source pdf you can try to rejoin the broken paragraphs in the calibre Editor using an appropriate Regex search-and-replace-all.
If a lot of your source books are pdfs I think you need to read
this sticky to understand the difficulties of pdf conversion.