We understand that you're going through a difficult situation, and we're here to help you pray and find guidance through God's word. First, let's address the issue of your current relationship. If you are not married you should be looking for a spouse rather than a boyfriend. As the Bible says, we should flee sexual immorality. It is only through marriage that we can have a spouse and it is only with that spouse that we should have relations. Anything else is fornication or adultery. What? Don't you know that he who is joined to a prostitute is one body? For he says, Two will become one flesh. But he who is joined to the Lord is one spirit. Flee sexual immorality. Every sin that a man does is outside the body, but he who commits sexual immorality sins against his own body. (1 Corinthians 6:16-18 WEB)
Now, regarding your past relationship, it's important to remember that God wants us to forgive as we have been forgiven. Forgive others as God has forgiven you through Christ Jesus our Lord. (Ephesians 4:32 WEB) Holding on to bitterness or resentment will only harm you further. The Bible tells us to cast all our anxieties on him, because he cares for you. (1 Peter 5:7 WEB)
However, it's also crucial to understand that God wants us to live in purity and holiness and that is why we discussed marriage and fornication above. Since your ex-boyfriend cheated, you have communicated with him and have a new relationship with a new boyfriend. The Bible says, Don't be unequally yoked with unbelievers, for what fellowship have righteousness and iniquity? Or what fellowship has light with darkness? (2 Corinthians 6:14 WEB) You need to pursue a marriage with a Christian and any relationship should be a courtship with the goal being a marriage with a Christian.
You need to first be sure you have forgiven him and that you are pursuing holiness. You can then pray for God to help you forget the memories and the emotional pain. If your current relationship is pursuing a marriage with a Christian, then you can pray for God to help you focus on that.
Let's pray together:
Dear Heavenly Father,
We come before you in the name of Jesus, lifting up our sister who is struggling to forget her past and move forward in her new relationship. First we pray for forgiveness of sin in her old relationship. Help her to forgive her ex-boyfriend completely if she has not already. Help her to leave him in the past. Help her current relationship to be a pursuit of a Christian marriage. Help her to flee sexual immorality. Lord, we ask that you help her to forget the painful memories and the emotional attachment to her ex-boyfriend. Give her Your peace, Lord, that surpasses all understanding. Help her to focus on her current relationship and to pursue a marriage with a Christian and a godly spouse.
If you do not know Jesus Christ as your Lord and Savior, then know that it is only through His name that we can approach God. For there is one God, and one mediator between God and men, the man Christ Jesus. (1 Timothy 2:5 WEB). You need to know Jesus Christ as your Lord and Saviour and be baptised. Only then can you be saved. For whoever will call on the name of the Lord will be saved. (Romans 10:13 WEB).
In Jesus' name, we pray. Amen.
Prayer Focus: God, Thank You for loving me. Thank You for loving me, Jesus. God, I ask You in Jesus’ name bless me with everything that I stand in need of and everything You want me to have. Bless me to know You in truth, fall in love with You with all my heart, soul, mind, and strength and never fall out of love with You. Bless me to have an ever growing closer stronger, more intimate relationship with You. Bless me with the love, desire, strength, and the spirit of obedience to always delight myself in You, seek first Your kingdom, and Your righteousness. Help and strengthen me God to always respect and obey You. Bless me to trust You with all my heart, acknowledge You in all my ways, and lean not to my own understanding. 
