Try to conduct an "exit interview" with the person who bailed. That way, you can find out why they bailed; if the reason is something that you can address, you'

Try to conduct an "exit interview" with the person who bailed. That way, you can find out why they bailed; if the reason is something that you can address, you'll do better next time. As others suggested, find another co-founder. Having equal levels of co-founder enthusiasm is the exception, rather than the rule, so don't be discouraged.