I am currently in an "exclusively dating" relationship with a guy, though we are not yet officially together. My problem is: we engage in masturbation over the phone because we both agree to not having pre-marital sex. Whenever we do this, I feel guilty afterward. One time I prayed to God and promised Him that we would not do it again until we're officially together. However, we've done it three times after I made the promise. I always feel bad and I always ask God for forgiveness, and I told Him that when we are already together officially, we won't be doing it for three months to make up for the three times we did it. I don't want to do it again because I promised God that I would not do it. I also feel disappointed in myself and I feel that God is angry at me right now.
How can I overcome the temptation not to do it again?
What you and your boyfriend are doing is really creating homemade pornography for each other. "For this is the will of God, your sanctification: that you should abstain from sexual immorality; that each of you should know how to possess his own vessel in sanctification and honor, not in passion of lust, like the Gentiles who do not know God" (I Thessalonians 4:3-5).
"But fornication and all uncleanness or covetousness, let it not even be named among you, as is fitting for saints; neither filthiness, nor foolish talking, nor coarse jesting, which are not fitting, but rather giving of thanks. For this you know, that no fornicator, unclean person, nor covetous man, who is an idolater, has any inheritance in the kingdom of Christ and God. Let no one deceive you with empty words, for because of these things the wrath of God comes upon the sons of disobedience. Therefore do not be partakers with them" (Ephesians 5:3-7).
Your conversation is about fornication. Your imagination is lust for fornication. "Let us walk properly, as in the day, not in revelry and drunkenness, not in lewdness and lust, not in strife and envy. But put on the Lord Jesus Christ, and make no provision for the flesh, to fulfill its lusts" (Romans 13:13-14). While you are not actually committing fornication, how are the things you are doing any less sinful?
Even if you were "officially" dating, which really has no meaning, it would not change these things into something right. Sex is only allowed within a marriage. You two are not married, so sex and the things that lead up to sex are not allowed.