You need to trust your instincts swimjim - sure sometimes one partner needs a little nudge to make the final committment but not an ultimatum. It takes a certain type to give one and usually they will use that technique when they want their way again in other problems.

Marriage and engagement should be a joyful time where both people are certain. She wanted a wedding in August but you weren't even engaged yet lol. Usually a couple gets engaged, then they discuss dates. If she does a 180 and decides to back off but still be a couple, then you might reconsider getting back together. The problem is, she feels she's ready for marriage and that will continue to be a factor in the relationship. If you're not ready, that's okay. Unfortuantely when a person gives an utimatum, it doesn't leave much room for 'in betweens'.