Thanks for contacting the Trevor Project. Relationships are tricky, no matter your sexual orientation. Sometimes we think we want one thing; but then it turns out our hearts want something else. At your age; this is completely natural and normal.
In relationships it's important that both people are happy and that they both have feelings for one another. It becomes a struggle in a relationship when one person is more committed than the other.
People sometimes stay in relationships long after the feelings have fizzled out. This is because they are afraid of either hurting the other person's feelings or they are worried about being alone. Unfortunately, when this happens, the other person most of the time ends up getting hurt anyway. Rather than drag a relationship on; it's better to move on. In the end, the both of you will be happier. You clearly have feelings for someone else; so it's only fair that you let him free. That way you won't miss out on your chance to be with your best friend; and you won't have to hide your feelings.
Hiding your feelings and secret kissing is tough. You're not only hurting you're boyfriend; but your hurting yourself as well; by not being true to yourself. Yes, your boyfriend will be upset; about the secret kissing with your best friend; but the truth always sets you free. Now, you may be worried about maintaining your friendship with him; if you end things. It is true that it's tricky to be friends with someone after you have dated them. But with time things do get easier and better. It is possible to be friends with an ex.
Be honest with yourself and your boyfriend. You'll both be happier; and you'll both be able to be with someone who wants to whole heartily be in a relationship with you; a hundred percent. We all deserve someone who loves us.