In mixing, I'm only using Sennheiser hd438 as my earphones, I believe this is not the appropriate to be used in mixing. What headphones would you recommend that's affordable?Regarding Sennhesier HD 438, I don't know the frequency response of your cans but when mixing you tend to look for Headphones that have a flat frequency response ( though in an ideal world there is no such thing as perfectly flat )..but look for those that are designed without color or boost on frequency response of your cans.
Regarding investing with good monitor speakers, take note that the full capability of your flat response speakers can only be utilized if you have an accurate room. In other words, your room should be acoustically treated. Otherwise, your monitor will just be lying. It will speak depending on the acoustic of your room. This is the reason behind why professional studios are properly treated acoustically.
This was also my mistake then when I knew nothing about acoustics.
On the bright side of it, you can still mix with your own room with your speakers if you can master and adapt the acoustics of your room. Likewise with the frequency response of your monitors.
At any Audiophile branch, you could buy the Shure SRH440. It's around 4k.