I say tehillim every day, and have been reciting it in Hebrew and then reading it in English. I started doing this in order to understand the tehillim, and it has now become habitual – I’ve been doing it this way for months, despite now understanding the words on their own.
I know there is no halachic requirement to read the English (aside from understanding our prayers), but because I seem to have taken it upon myself as a way to better perform the mitzvah, have I somehow made a neder for myself? And if so, do I need to do hatarat nedarim in order to stop performing the mitzvah this way?
I would still be reciting the tehillim, but don’t feel as though I need to be reading it it both languages anymore.
You do not have to be matir neder. Tehillim and other tefillos should really be said in lashon hakodesh, because there are a lot of hidden things in the Hebrew text. Therefore, although it is permitted to say them in English when needed, we are not going to call it a devar mitzva, in the sense that it would be considered a neder. If anything, now that you aren’t saying it in English you have time to say more Tehillim in Hebrew. If you will add to the amount hat you say, make sure to say bli neder.