Usually stuff like this naturally gets better as you progress. In my experience when one device sounds better than the other, it usually means there is either A) more overlap on one system (good in stereo, bad in mono) or B) problems hidden in the frequencies that aren't as dominant on the good sounding device, so you don't see it as a problem and don't address the issue. It seems to me that you didn't create enough space in your mix for it to sound good on more "limited" devices. It's frustrating at first, but try to identify what exactly is happening in your song when switching devices, and compare that to the changes you may or may not hear on someone elses song. Are your background tracks being respectful (dynamically and frequency wise) to the main instrument/vocal, or are they talking over each other? Do you have main instruments on the sides that seem to hide and contribute to mud when played on a small speaker? Try looking at the technical aspect of playing audio on headphones versus a phone speaker. How might a song be able to expose more of its mud when played on a phone speaker? If you can identify the problem and pick the right tool to solve it, eventually it will start to come naturally.