I am a Christian and have a Muslim boyfriend. I am separated, since 14 years ago. I know my boyfriend will not marry me but I do love him, and I promised that he will be my second and last men in my life. Soon I am ready Inshaallah, I want to enter Islam but in case I enter Islam and we meet again, is it haram to sleep with him again?
Sex outside of marriage is not permissible in both Islam and Christianity. The primary scripture of Islam, the Quran strictly prohibits it:
And do not approach unlawful sexual intercourse. Indeed, it is ever an immorality and is evil as a way. [Quran 17:32]
It is also prohibited according to the Bible:
27 “You have heard that it was said, ‘You shall not commit adultery.’[e] 28 But I tell you that anyone who looks at a woman lustfully has already committed adultery with her in his heart. 29 If your right eye causes you to stumble, gouge it out and throw it away. It is better for you to lose one part of your body than for your whole body to be thrown into hell.
-- Matthew 5-7
You asked: I want to enter Islam but in case I enter Islam and we meet again, is it haram to sleep with him again?
Yes, it is haram for you to sleep with him again (haram means prohibited). And if you don't convert and stay a Christian, it is still prohibited according to the clear verses of the Bible. If you want to be intimate with him, marry him.
Sex outside of nikah (marriage contract) is a major sin, is haram and is a punishable offence under shariah law.
I have no background knowledge of your specific circumstances ... but for reference purposes there is something called Misyar (also see islamqa.com) in Sunni sect and Mut'ah in Shia sects that you may research upon and see if you two can agree upon something like that should the situation arise.