You need to indeed end it with this girl. This relationship is nothing but evil- and virtually certain to be a lot more sin and trouble yet if you let it continue. Including that and beyond that the thing which you can and must do is submit to God and break your commitment to your flesh in repentance- and you must then walk daily going forward maintaining that commitment which is essentially carrying your cross in identification with Christ in His death to sin and living unto God. You can do this because God provides strength to overcome sin and to live acceptably to Him to those who do the above. There is also mercy available through Christ's High Priesthood where those who do the above (and only those) can be cleansed by Christ's blood for their past sins and where they yet fall short in the present. There is a never ending supply of grace through Christ for those who submit to God and wholeheartedly seek to please Him to the point of death to self and suffering for the Truth of His Word. He has made precious promises on His end of His covenant which make it possible for Him to carry you to the end faithful, pure, and blameless as you trust Him and do your end of the covenant. The facts that He does the harder part, that He is faithful, and that His demands for us are reasonable make it possible and realistic for us to our part- if we will only believe His Word and embrace the suffering that is involved with acting rightly in accordance with his Word.
John 12:24-26: "24 Verily, verily, I say unto you, Except a corn of wheat fall into the ground and die, it abideth alone: but if it die, it bringeth forth much fruit. 25 He that loveth his life shall lose it; and he that hateth his life in this world shall keep it unto life eternal. 26 If any man serve me, let him follow me; and where I am, there shall also my servant be: if any man serve me, him will my Father honour."
James 1:21-27: "21 Wherefore lay apart all filthiness and superfluity of naughtiness, and receive with meekness the engrafted word, which is able to save your souls. 22 But be ye doers of the word, and not hearers only, deceiving your own selves. 23 For if any be a hearer of the word, and not a doer, he is like unto a man beholding his natural face in a glass: 24 For he beholdeth himself, and goeth his way, and straightway forgetteth what manner of man he was. 25 But whoso looketh into the perfect law of liberty, and continueth therein, he being not a forgetful hearer, but a doer of the work, this man shall be blessed in his deed. 26 If any man among you seem to be religious, and bridleth not his tongue, but deceiveth his own heart, this man's religion is vain. 27 Pure religion and undefiled before God and the Father is this, To visit the fatherless and widows in their affliction, and to keep himself unspotted from the world."
1 Corinthians 10:13-14: "13 There hath no temptation taken you but such as is common to man: but God is faithful, who will not suffer you to be tempted above that ye are able; but will with the temptation also make a way to escape, that ye may be able to bear it. 14 Wherefore, my dearly beloved, flee from idolatry."
You NEED to fear God, depart from evil and not be ashamed of Christ!
Proverbs 16:6: "By mercy and truth iniquity is purged: and by the fear of the Lord men depart from evil."
Matthew 10:32-39: "32 Whosoever therefore shall confess me before men, him will I confess also before my Father which is in heaven. 33 But whosoever shall deny me before men, him will I also deny before my Father which is in heaven. 34 Think not that I am come to send peace on earth: I came not to send peace, but a sword. 35 For I am come to set a man at variance against his father, and the daughter against her mother, and the daughter in law against her mother in law. 36 And a man's foes shall be they of his own household. 37 He that loveth father or mother more than me is not worthy of me: and he that loveth son or daughter more than me is not worthy of me. 38 And he that taketh not his cross, and followeth after me, is not worthy of me. 39 He that findeth his life shall lose it: and he that loseth his life for my sake shall find it."
Remember: Titus 1:16: "They profess that they know God; but in works they deny him, being abominable, and disobedient, and unto every good work reprobate."
Revelation 21:7-8: "7 He that overcometh shall inherit all things; and I will be his God, and he shall be my son. 8 But the fearful (i.e. cowardly), and unbelieving, and the abominable, and murderers, and whoremongers, and sorcerers, and idolaters, and all liars, shall have their part in the lake which burneth with fire and brimstone: which is the second death."