Melodic key wheel would be a good idea looking into. To further explain some above comments, when mentioning your keys clashing, means you are not in the same key or in harmonic 3rd or 5th steps. Therefore, the reason you have clashing.
It is very simplistic to get a straight ahead beat to mix. However, first begin your mixes with them being in proper keys. Therefore, they will seamlessly and melodically be in the same key and you won’t really notice that your mixing two songs together, except you will know they are two separate songs, for example the mash-up.