I learned two languages during the "critical period". The first language I was exposed to was my "mother tongue", and so I feel most comfortable speaking in it. I went through English schooling, and find it easier to write in English relative to my mother tongue.
"First language" isn't exactly a well-defined technical term of linguistics, so it can be used literally, as the first language that you learned as a child. I know someone who started life speaking Polish, but the family moved to the US and his Polish kind of lapsed, so he's not really fluent. "Mother tongue" is likewise a problematic term since it can be used to mean "the language that my mother speaks", relevant in multi-ethnic communities where your ethnic identity comes from your father, but you also learn the language of your mother's side of the family. (This can lead to people thinking that they "can't speak their own language" if they don't become fluent in their father's language).
We generally disregard ability to write as indicative of anything, since many languages in the world are not written, either at all or else very much (that is, there aren't any or many publications in the language and at most it's just written informally in letters to home). So if your first language were Gitonga you might speak that language better than English, but you might struggle with writing Gitonga and be better at writing in English, because you didn't have much experience with writing essays in Gitonga. Your description of the situation suggests to me that your "first language" is not English, but maybe you really are equally conversationally fluent in English and X. I take "most comfortable" to be a self-assessment of oral fluency (favoring X); now maybe you can decide what your "native" language is.
You do not mention what you specify as
mother tongue but whatever it is, will be your
L1 Language, your native language.
Reading and writing is a model of language that was created after humans were speaking.
Reading and Writing are independent of each other.
Whatever language you learned first to speak, will be your native language.
native language and
mother tongue and
L1 Language are equivalent.