I'm not sure exactly what kinds of thoughts you are having. Feeling strong sexual desire is quite normal, and I don't think that Scripture calls that wrong. Dwelling on detailed thoughts of specific sexual acts ... not a good idea, it will make it much harder to do what is right.
The word 'lust' in many Bible translations simply means a strong desire, there is no special Greek word for sexual desire here. Jesus uses the same word, for instance, to say that he eagerly desired to break bread with his disciples.
In Matthew 5, Jesus says this:
"You have heard that it was said, 'Do not commit adultery.'
But I say to you that whoever looks at a woman to desire her has already committed adultery with her in his heart.
I don't think your sexual desires for your boyfriend are the same thing Jesus is talking about. Adultery involves someone else's spouse, so that's what Jesus is talking about here. And the word 'desire' or 'lust' is the same one used for "thou shalt not covet". So Jesus is saying that it's not only wrong to have adultery with your neighbor's wife, it's wrong to covet her, to dwell on thoughts of having sex with her.
But I do think it's a good idea to keep your mind from going too far in directions that are not appropriate, because where your mind goes, the rest of you is likely to follow. It's really hard to decide not to think about something, but it's surprisingly easy to acknowledge what you are thinking and feeling and decide to think about something else. Don't fight what you think or feel, let yourself acknowledge these things, realize when it's too much, and have some other things to think about. Make a list of things to daydream about or work on so you have it handy if you need it ....
Hope this helps ...
