The Sign of Pisces is known as the cosmic sponge of the Zodiac, meaning that we easily absorb other people's negative energy; especially if as always, we are trying to help them out.
You don't have to stop being yourself, just protect yourself from this. Here are some ways:
Crystals - Carrying or surrounding yourself with crystals like amethyst, black onyx, obsidian and tiger's eye will cleanse your aura.
Mantras - The power of words has a powerful effect on our minds and bodies. If you are close to someone who is upset or sad, mentally repeat positive phrases like "May I be protected" or "May I be happy".
Cleansing baths or showers while meditating will wash away the negative energy that you might have absorbed from others, but you must do it with intention.