My boyfriend and I have done some sexual things. We love each other, and I know I'm going to marry him someday. We've been struggling to stop, but I think the time has come to give up these things, as I know that it is not right in the eyes of the Lord. Are we going to hell?
I can't predict any person's eternal destiny because God looks at who you are and not who you were. I have no idea if you will change or if, upon changing, you will remain faithful to God. What I can tell you that dying in your sins will lead to condemnation. "Do you not know that the unrighteous will not inherit the kingdom of God? Do not be deceived. Neither fornicators, nor idolaters, nor adulterers, nor homosexuals, nor sodomites, nor thieves, nor covetous, nor drunkards, nor revilers, nor extortioners will inherit the kingdom of God. And such were some of you. But you were washed, but you were sanctified, but you were justified in the name of the Lord Jesus and by the Spirit of our God" (I Corinthians 6:9-11). While you talk about stopping, you haven't. That tells me that you really haven't changed your mind about your sin. You feel justified in doing these things because you claim you are in love and that you plan to marry.
Plans are good to have, but plans are not a guarantee. "Come now, you who say, "Today or tomorrow we will go to such and such a city, spend a year there, buy and sell, and make a profit"; whereas you do not know what will happen tomorrow. For what is your life? It is even a vapor that appears for a little time and then vanishes away. Instead you ought to say, "If the Lord wills, we shall live and do this or that." But now you boast in your arrogance. All such boasting is evil" (James 4:13-16). A plan to get married doesn't change the fact that you are not married and having sex when you are not married is still fornication.
Love isn't a feeling. People in love have feelings for the one they love, but the feeling isn't love. "Love suffers long and is kind; love does not envy; love does not parade itself, is not puffed up; does not behave rudely, does not seek its own, is not provoked, thinks no evil; does not rejoice in iniquity, but rejoices in the truth" (I Corinthians 13:4-6). Notice that love doesn't take pleasure in sin, yet that is what you have been doing. It isn't love to encourage another person to sin since no one would want a loved one to end up in hell.
Since you know you are not doing right, why are you not changing? It is past time to get right with God.