The sort of commitment and union you'll both want to create together will be at odds with where you should head as a couple, which will make it difficult to make any kind of progress together. When you're feeling good about your relationship and your shared commitment, you'll feel as if you're stagnating in terms of forward progress. And when you're moving forward in the direction that's right for you as a couple, you'll both feel as if you're not getting what you want from this union.
This will be a frustrating duality, but one that offers you a growth opportunity, if you'll take it. You'll need to work together on creating a balance between commitment and forward movement as a couple. It's not impossible, and putting time and energy into solving this problem will be a growth experience for both of you. Ultimately, you may have to accept that your partnership needs will be met only some of the time, and that you'll progress as a couple in fits and starts at best.