I would downplay 'market rate'. That's fine for commodities. Are you a commodity? You are in a certain role and that does put you in a range of how much value you are adding/are capable of adding (although at a startup this would be a wider range than an established co.) Your only justification for an increase is that you are adding more than you're currently being compd for.
There's other factors too, perks, travel demands, how fulfilled you feel, how much you like/hate your boss. You can determine what you are happy with and accept that, but if you can't get it, think about leaving if you can get more elsewhere. But don't stress too much about - am I getting 100% of what I could get.
Know that - that doesn't define your worth as a person - and there are many people making much less than you, and many making much much more. Some of them incompetent. Are you happy is the question.
There's other factors too, perks, travel demands, how fulfilled you feel, how much you like/hate your boss. You can determine what you are happy with and accept that, but if you can't get it, think about leaving if you can get more elsewhere. But don't stress too much about - am I getting 100% of what I could get.
Know that - that doesn't define your worth as a person - and there are many people making much less than you, and many making much much more. Some of them incompetent. Are you happy is the question.