Let me try and answer your questions in the following way.
The Bible doesn’t say that temptation is a sin, but rather when we do give into temptation, that’s when it becomes a sin. Jesus was tempted to sin. The gospel accounts in Matthew and Luke explicitly describe Jesus being tempted by the devil head on (Matt 4:1-11 and Luke 4:113). Throughout the rest of the scriptures we read about the subtle devious temptations Jesus continued to face. Yet, Jesus stood strong. He did not give in. In fact Hebrews 4:16 talks of Jesus being tempted in every way that we have and yet did not sin - that is why he does know what you are going through.
The Bible also tells us we do not face any temptation that others haven’t faced before and that God will never allow us to be tempted beyond what we can bear. Rather when we are tempted He will provide a way out so we can stand strong (1 Cor 10:13).
So when you are tempted there are two important things to do - first ask God to help you, then do anything you can to remove yourself from the temptation. It may not be easy but its the way to do it.
Sexual temptation is particularly difficult and I am glad you are concerned about it.
Sex is a special and precious gift from God, given to us to be experienced within the context of a loving marriage relationship. It is a powerful activity that brings a couple together and unites them very intimately - they become “one flesh”. Satan tempts us to think that if we don’t indulge in it with our boyfriend/girlfriend then we are somehow missing out. But the truth of the matter is that sex under such circumstances does not really satisfy nor lead to a better relationship (despite what you see on TV). It can lead to serial relationships and long term consequences.
In one way you cannot stop being tempted to have sex with your boyfriend. It is a “natural” reaction of the attraction you have to him. But for the sake of godliness you need to resist it and the best way to do this is to pray and avoid all situations where your temptation can lead to other things. Try to avoid being alone with him for instance; rather meet with him in circumstances where there are people about. Talk to him about it. If he loves you and shares your concern then you can help each other.
Stand strong! You can in God’s strength.