Well I'm proof (and I could mention a few others) that you can truely get someone out of your system. Both head and heart.

Besides, wouldn't it be unfair to my bf if my ex was still monopolizing space in my heart???

Part of moving forward means cutting ties with your past.

I don't act like I couldn't care less about my ex, I don't act as if he doesn't matter. He just isn't a factor any more, he no longer features in this film of mine, so he isn't given any "act".

The only reason I ever reference him is when prompted by a discussion such as this one where I think it will benefit the reader / listener, or on the rare occasion when someone says "I saw your ex today he's looking good/ bad/ hasn't changed a bit" and I go "sooo?" And then we proceed to have a conversation I'm actually interested in having.

Maybe you mean "heart" differently though and I'm misunderstanding, or maybe you are referencing amicable break-ups. Either way, all I'm saying is that your thought is very inaccurate.