One good rule of thumb about composing is, if you’re having trouble making a part “fit” it usually means that it isn’t the right part to begin with. Once you (finally) arrive at that conclusion there are a few options, including further modifying what you’ve written or starting again from scratch. The idea of starting again raises interesting questions (N.B. it does not “beg” the question) about just how rasa the tabula can be.
You can’t “un-hear” what you wrote/played before, so anything new that you come up with is in one sense a variation, even if the parts have nothing in common. At the same time, you can become so saturated with the original part that it just fades into the background, which can free-up some creative space.
Time away from the music usually works best, even if it’s just a fresh start the following morning (and a new set of strings).