You will find it more easy to open yourself towards some persons,to others you will not.Usually the persons who you open to are the ones who naturally express themselves more easy.But there is more to it,I think it's mostly related to the values you share.
Now I don't know much about this gf of yours,but from her reaction
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She accused me of being a controlling partner and how I don't want her to spend time with her friends
well i would myself find it hard to open to her,since she doesn't encourage this behavior (from the contrary...)
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During the past few years, I thought I was doing well on expressing myself.
That's good,you keep improving yourself.
It's not your fault that you can't express yourself so much,you do it through actions anyway,it's her insecurity.So don't reassure her,rather turn it on her "why can't you trust me ?"