I'm 18years old and I'm in relationship with a girl and sometimes I don't want to do these things but every time she says to me For sake of Allah be with me , I repent what I have done in past, can I lie to her to get out of this haram relationship?
You can deny her call for zina without lying. You can be straight forward with her, you can say my faith do not allow sexual relationship outside of marriage. This can be testing times and if you have faith (imaan) you should stay away from forbidden things.
In fact if you refuse her call for zina, you will also be rewarded like mentioned in this hadith
Yahya related to me from Malik from Khubayb ibn Abd ar-Rahman al- Ansari from Hafs ibn Asim that either Abu Said al-Khudri or Abu Hurayra said, "The Messenger of Allah, may Allah bless him and grant him peace, said, 'There are seven whom Allah will shade in His shade on the day on which there is no shade except His shade:
a just imam, a youth who grows up worshipping Allah, a man whose heart is attached to the mosque when he leaves it until he returns to it, two men who love each other in Allah and meet for that and part for that, a man who remembers Allah when he is alone and his eyes overflow with tears, a man who refuses the approaches of a noble, beautiful woman, saying "I fear Allah," and a man who gives sadaqa and conceals it so that his left hand does not know what his right hand gives.' " https://sunnah.com/malik/51/14