Congratulations on making your first cartridge! It sounds like you followed the right steps in pressing the hash and decarboxylating the rosin before filling the cartridge. However, the issue you're experiencing with the density and difficulty in vaping could be due to a few factors.
First, the viscosity of the rosin may be too thick for the cartridge you're using. Some cartridges are designed to work better with thicker oils, while others work better with thinner oils. You may need to experiment with different types of cartridges to find one that works well with your particular rosin.
Another possible issue is the temperature at which you're vaping the cartridge. If the temperature is too low, the rosin may not vaporize properly, resulting in a lack of smoke or vapor. Try increasing the temperature slightly to see if that improves the vaping experience.
Finally, it's possible that the rosin itself is not of the highest quality or purity. If the rosin contains impurities or contaminants, it may not vape as well as a higher-quality product. You may want to consider using higher-quality hash or rosin for your next batch to see if that improves the vaping experience.
Overall, it may take some trial and error to find the right combination of hash, rosin, and cartridge to produce a high-quality vape experience. Keep experimenting and adjusting your technique until you find what works best for you. Good luck!