Ah, yes...the reddit arbiter of advice.

I don't know what to do with "don't believe anyone but me" part or the last sentence because I don't see those anywhere.

But I'll address the "fix yourself" part, as it's a direct quote.  Most people date, and decide if they want to marry this person or not.  Getting married or not in general is a question they've already answered with "yes".  He, for some reason, has answered "no" to this question.  His reason is an age-old trope about not needing to prove his love via marriage, which is nonsensical as that's not the point of marriage.  Marriage is not a proof of love statement.  It's a commitment of love statement.  This is the issue his GF has with him - he's unwilling to demonstrate his commitment of his love.  It's strange, because he's willing to actually live has a married person - just without the title.  Therefore, he has a skewed view of marriage.  This is not unusual, as a bad family experience often gives people this view.  However, in the end, we are all responsible for our own lives, even if it was inflicted upon you.