1. At this early stage of the business, the core asset is really the co-founding team and their relationship. Conflict is very much normal, but how you resolve it is important. You should be able to articulate your co-founder's concerns. For ex. she might feel the pressure to show "progress" to your investors.
2. It takes a lot of time and energy to build a good working relationship. Moving onto something else might seem like a good idea at the moment, but you could be right back with the same issue with another co-founder.
3. If you do decide to move on, end the relationship as amicably as you can. Note that this is not necessarily the "quickest" route, but it'll help keep your reputation intact.
Edit: spelling.