Don't think of her boyfriend as an obstacle. The fact that you are shy means you'll have a hard enough time talking to her about your feelings without worrying about the boyfriend issue. If she likes you, SHE'LL be the one to end things with her boyfriend, no one else.
What makes you think she is attracted to you? Are you sure? You need to be sure. Since when was being attracted to someone inconsiderate? If you're too hesitant, you'll lose out and probably regret it.
You can't force her to break up her relationship, but you can give her something to think about.
You must talk to her. Say to her (for example) that the last thing you want to do is come between two people in love, but if there was even a chance that she might feel about you the way that you feel about her, you'd always regret remaining silent about your feelings.
Now she'll know for sure how you feel. If you misread the situation and she isn't attracted to you, she will hopefully be flattered and let you down gently. If she IS attracted to you, she'll know exactly how you feel and will be better able to decide what to do about her existing relationship.
If someone is considering breaking up their relationship because they like someone else, they are MUCH less likely to take the plunge if they are unsure about how the person they like feels. They might end up with no one. If you tell her, she will know! She'll be able to make an informed decision.
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