Tricky situation! Here are my observations based on what you described: The very fact that you're thinking if it would be fair for you to build the business o

Tricky situation! Here are my observations based on what you described: The very fact that you're thinking if it would be fair for you to build the business on your own while owning only half the business suggests to me that it's only the incentive of owning more of the business (or rather the lack of such an incentive) that is preventing you from actually doing it on your own. You already know you will be able to do it alone deep inside or so your style of writing suggests. That said, I would suggest you speak to your co-founder about how the business should grow in her absence. That should get the ball rolling and set the tone for one of you to bring up this topic.