I guess I was in a similar situation. In Jul/Aug'11, my partner of 10 years was diagnosed with GBM and passed away in Sep'12. I'm a loner and private person. At the time, none of my friends and family knew that I had a partner. I didn't have any close friend when I had to deal with this. I was pretty much on my own. My partner knew about this so we made it clear right from the start that we would be opened and honest to each other, expressed our fear, and shared our thoughts. He understood we were in this battle together, not just himself. Fortunately, my manager at work was also extremely helpful and understanding. He worked in a different office but regularly called me to discuss work and to see how I was in general. I think it's important to have someone to talk to. I talked to people on this forum and a couple other cancer forums. I created a blog on this site as a way to let go of my feelings and any frustrations I had which I found very helpful.
As for feeling that you will be on your own after all this, I believe you will find a way. Someone gave me a very good suggestion at my partner's funeral - say 'Yes' to any invitations! Be open-minded and reach out to people you know, any friends you have. It doesn't matter how distant you are to these people. You never know what will happen next. A loner such as myself, somehow, found a way to connect with a couple of girls from university who migrated to Australia like me. We studied at the same department but were never really friends. Now I'm happy to say they are my friends and one of them has become my best friend actually. There's always a bright side of life 🙂