Hello, I'm 25M

I've been living my whole life in a useless way. I feel I haven't collected enough experiences in life so I could share them in a regular conversation and I don't know what to do about it now. It happens in every social interaction I have. I just don't know how to talk to people properly and it's extremely awkward. It bothers me to the point I feel stupid and it drains my will to do anything else throughout my days. Being in silence is fine for me, but I feel I should talk more, especially to my gf. 
We get along very well and have many things in common but sometimes I have nothing to say and it feels just weird because she's younger than me and it's way more intelligent and she definitely knows how to express herself. She's an artist and a future psychologist.
I feel she's going to get bored of me and eventually dump me (or find someone more interesting) if I'm not enjoyable enough. 
That's how I feel in most social interactions. But I think it's not a relationship problem, it's some kind of social/communication issue I developed during my whole life. Maintaining a conversation is not a problem at all for most people, but it's for me. I stare at the person and my mind goes in circles trying to pop out some random fact or joke. But I just can't. 
I've never been encouraged to express myself in a way everyone does. I spent my life just playing videogames, studying and having minimum social experiences. Is that some kind of blocking that make my mind empty all the time? Despite the fact that I have a comfortable life, a partner who I'm in love with and a slightly promising future, I just cant stand myself to the point I hate who I am. I wish I was someone else just to be a regular person and not an empty one. 

I don't know if I'm making any sense but I don't know what to say. It's just like my head 24/7, empty. I can't think of a cool story to make someone laugh about it. Been going to a a therapist for the last 5 years and got diagnosed with anxiety and light depression. Most of the time I'm silent and trying to figure out something nice t say but I simply don't have. Should I accept it? I don't have social skills, I feel dull and dumb. I feel I'm not a interesting person at all, totally disposable and it's starting to come out stronger and affecting my relationship and life in general. If I don't figure out something I'm gonna spend my days in my room doing nothing and rotting. I feel awkward by just typing this text because it does not make too much sense and I don't know to to express myself