I don't want to break it to her that I've got a girlfriend. I'm gonna keep this going.
This is quite a shock. Why don't you want to mention that you have a girlfriend?
I don't want her to leave me.
It's disrespectful to have two girlfriends at the same time if you've committed to being monogamous. You should break up with one of them and be faithful, because lying to people and cheating on them is an awful thing to do.
I have no plans of ending this relationship with my girlfriend or with her either.
Having a gf and another girl on the side is disrespectful to both women. You will need to decide on who you want to be with or risk losing them both.