How do I make friends without being in school?

I am 20 years old, and am a full time realtor. My business is pretty much exclusively with people that are at least 10 years older than me. All my friends from high school went to college and I lost touch... except for 2 friends that are in college out of state, so I really only see them holidays and breaks. They're great friends, but I only see them a handful of times each year and our interactions are fairly superficial.

I have nothing in common with anyone at work/any clients. All my relationships there are strictly business. I feel like I have 2 personalities: my "work" face, and my real face.

The lack of friendships/real human connection in my life is taking a toll on my mental health. I have a girlfriend who I have seen since high school, and part of me feels like the only reason I am still with her is because without her I would have nobody. She doesn't do the whole "diving deep" thing. She feels more like a friend at times than a girlfriend.

What should I do? I have considered getting a part time job just to meet people my age, even though its completely against my current goals. Do I really need friends to be happy? How do I find happiness if every single day is spent alone?