Financially speaking this is your baby, but it sounds like you never set out what the ownership / responsibility split would be from the beginning. This is your biggest problem, and is what is causing you to resent the arrangement, not necessarily the person. I'm not sure if your startup is making money yet (you mention salary) but I'm guessing it's 50/50 all the way.

Someone here has suggested talking to a lawyer, but that's a big step for friends especially as this is the start.

Another suggested getting in a third party. This is a good start, and you should offer some small equity in the process. Firstly that allows you to renegotiate your share, secondly, it changes the power balance, and thirdly (and perhaps more strategically so keep this as the ace hand) you can plant the seed of walking. This last point is very important because it communicates that you are prepared to take a project that you appear to have built, elsewhere, and that they will end up having to do everything if they want to continue as competitor.