She's sad I'm leaving her..

My boyfriend and I moved in with my mom and have lived with her for about 3 months to save money to get an apartment. We finally found the apartment we're looking for and I told my mom the news. She tried to hide it but she started crying. My mom gets depressed when she's alone because my stepdad works often and she's lonely by herself. I feel horrible and I don't know how to make her feel better.