I'm an introvert and I have trouble making new friends. I feel lonely?

I am a 28 year old, living with my girlfriend. She is a real extrovert - she started a new job not so long ago and by the end of her first shift she knew everyone by name, had nicknames for all of them, was joking around with them and having the time of her life. Me, on the other hand, I find it very difficult to make new friends; I've been at my job for just over a month and would say that there is no one I would consider a friend there - just work colleagues. I just find myself unable to hold conversations so I don't really get into them. The friends I do have have all either moved far away so it's difficult to see them, or have settled down, got married and started a family of their own, which I understand will now be their new primary focus. My girlfriend is currently on a bit of a training period which means she is working almost every hour under the sun, so we don't have much time together as my day off and hers do not coincide, and due to her long hours, when she is home she is sleeping (she regularly pulls 15 hour shifts and doesn't leave work until 3am many days). I work only evenings, so I am home during the day and I find myself feeling a bit lonely. As I said in the title, I don't make friends very easily and everyone I am close to seems so busy these days. I don't even know what I am hoping to get from this, whether it's advice or just to let off some steam from feeling so alone.