Fragile, brittle nails is a very common medical problem. Fortunately, it is very rarely a serious medical problem. Sometimes, brittle nails can be a sign of a nutritional deficiency, such as iron deficiency. However, this should also affect the toenails. So, if you have normal toenails, then it is likely that some external factor related to how you use your fingernails is the problem rather than an underlying medical problem.
By far the most common cause of brittle nails is exposure to water. This is common in those who wash a lot of dishes, for example. Making sure to use gloves when ever immersing your hands in water can help. Similarly, multiple chemicals can damage the nails. These include nail polishes and removers, chemicals used in applying and removing nail gels, and the like.
The fact that your nails are turning a bit black at the ends suggests either that, due to their raggedness, they are catching and retaining small dirt particles or that there is a fungal infection settling in. It would be best to show your nails to your primary care doctor
to rule out this possibility. If a fungus is responsible, then your doctor may prescribe an antifungal medication.