I feel like I've hit a ghost. Is there any way to ward off evil spirits?

That's because you think too much. Think every day and dream at night! Maybe you miss her too much. If your grandmother really gave you a dream, then what she said should be true. There should really be food in that place. If not, it means you are imagining things. Don't think too much, relax yourself, everything will pass. If you are still not at ease, you can try to buy a mahogany pendant to carry with you. Since ancient times, there has been a saying in China that mahogany exorcises evil spirits, and many people in this line will also have things like mahogany swords around them, which shows that mahogany is still effective. The Buddha statue is ok, and ghosts are afraid of this more correct thing. If you really can't, you need to ask Zhong Kui, otherwise you need to find a psychic or something to help you!