How to make my minpin less agressive?
I have a 6-year-old Miniature Pinscher who is pretty aggressive. She seems to prefer being left alone most of the time. Occasionally, she'll come over and lie on my lap, but after I've been petting her for a while, she'll suddenly growl and bite me. She'll also bite me or other family members if we try to pet her when she's by herself. No one can even get close to her while she's eating; otherwise, she'll attack and bite.
Two years ago, my family got a Shih Tzu, and things got worse. My Shih Tzu is very playful and is always trying to interact with my Miniature Pinscher, which often leads to fights. My Miniature Pinscher was raised with my cat, who has since passed away. My cat was very calm, so I think my Shih Tzu's much higher energy level is not what she is used to
Unfortunately, I didn't know anything about dog training when I got her, so I probably made a lot of mistakes. Is there anything I can do now to improve the situation?