First, this isn't even a little bit strange! In fact, I'm sure you've heard the term "empty nest syndrome". What makes it different in your situation is that your mom has no support system besides you. I'm sure she didn't do this on purpose, but she's putting a terrible burden on you.
If you aren't leaving until next year, you have plenty of time to encourage her to develop a life of her own. Do some of the research for her and even offer to go with her (if applicable). Start by thinking of things she enjoys. If it's reading, find a book club. If it's cooking, see if there are classes at a nearby community college. In larger cities, restaurants will offer 6 week classes or whatever. It's so much easier to meet people when there's a shared interest. Heck, most cities have singles groups for older people. You're not trying to marry her off, lol. It's about her expanding her circle and her support system. Volunteering is another great way to meet people, and by definition, these are probably very good people. Another option: if your house is so big, maybe tell her to sell it and get into a healthy older community. She'll get away from all the memories, and she can be as active or reclusive as she wants.
Obviously, you can't force her into any of this, but if you keep trying and nothing works, when it gets closer to the time you leave, you need to inject some tough love. Tell her you've earned the right to pursue your dreams without worrying about her, and if she refuses to do any of this for herself, maybe she should consider doing it for you. It's fact she'll be happier with more going on in her life, but she still has to be the one to take those first steps.