That is an interesting question and a rather difficult one to properly answer. Typically when we roleplay, our imaginations tend to run wild and sometimes we feel like we are emerged within the universe we’ve created. Now the question and answer here is; how well are both of you describing your character’s and the world they are interacting with?
If you feel like you are not your own character, but your partner’s character, then are they doing a good enough job seperating themselves from your character or vice-versa?
Remember that roleplaying is like building a story; and typically that story is between one or more authors or narrator’s.
If you wish to truly seperate your character from the rest, then perhaps you need to set up better guidelines for the roleplay and the character’s themselves. Discuss how you want the character’s to play out; what their personalities might be, or what their appearance is.
Discuss how you’d like the other person to write; whether you prefer them to write in first person, third person, etc. Try to leave room for them to express themselves openly, because everyone wishes to enjoy the roleplay and if you become too controlling of it, no one will want to rp.
While you might always feel like you are in some way the other character’s within the roleplay, if you can seperate each character in some way, shape, or form; then perhaps you’ll feel more comfortable within your own character’s shoes.
Good luck, I hope my answer helps to some extent. Take care and happy roleplaying!