Hi OP, sorry to hear you are feeling lonely, it is the most awful feeling isn't it? I think if you've always felt this way, the relationship you have will probably exacerbate the feeling, however, it doesn't sound like it's you're relationship that's the problem.
Have you looked at your childhood? That's normally where everything stems from, and will probably be where you need start from to get to the root of your problem.
I too, feel lonely. It is deep within me, and I have resigned myself that I will always feel this way. I was in a relationship for 7 years, I was incredibly lonely in that relationship. Admittedly I wasn't happy at all, and I finished with him, but he wasn't the cause of my feelings of loneliness, it runs much deeper than that.
I can be surrounded by people, and still feel terribly lonely. I am aware of this, and try hard to acknowledge those feelings, but it's a work in progress.
I have low self esteem and self confidence, think nobody likes me, and always feel excluded. I joined a Meetup group when I split with my partner, I chat to everyone, and feel like I get on with them, but I watch others make friends, while I barely make acquaintances. I constantly question myself, what am I doing wrong? I have decided to distance myself, see if anyone actually notices my absence, I doubt it, so I will eventually leave the group. They're all nice people, I know it's me that has the problem. The group magnifies my issues.
I hope you find a solution to your problem OP, I would try looking for the root of it first. These isolated times are truly a time for deep thought and reflection. 