If youre not legally married, you should be filing single every year.
Since you are not married to that person you cannot legally file as either married filing joint or married filing separate. If you have a dependent relative living with you you may be eligible to file as Head of Household. Otherwise, file as single.
On the other hand, since Texas recognizes common law marriages, you MAY be considered married but you need to consult with someone who is familiar with Texas law on the subject and with the implications. If you actually WERE married last year then you still are unless you have gone to court to get a divorce.