As much as you might want it to be something else, the most likely reason that your back is rounded in forward bends is that your hamstrings are tight. Tight lateral gluteals could also restrict you, and of course, there is also the chance that there is some type of bony restriction as well. Sometimes the obvious cause is the cause. Whether you struggle with uttanasana or paschimottansana, consistent practice is the key. That doesn't mean that it needs to be intense practice. In fact, I would argue that many people try to open their hamstrings too quickly.