Sorry, but you're going to hear a bit of it. It's not your friends fault for caring enough about "the person you want to be with" to tell him, it's your fault for doing drugs and kissing him. Albeit in my opinion a kiss isn't the end of the world, some people think it is. You can't prove to him anything, and why should he believe it when you've been disloyal? It's not as if you can show him a video recording from the previous night, so tell him the truth, and let that be it. The more you flounder around trying to add in little details that make it 'crystal clear you're telling the truth' the more of a liar you'll look like.
I don't mean to be rude, but grow up. You can live without him. You lived before him, you can live after him. If things don't work out, they don't work out. Please don't let yourself become obsessed, wasting the best years of your life yearning after some boy when their are thousands out there that are probably a way better catch than him. Plus, if you do that, and he doesn't actually want to be with you anymore, you'll be putting him through years of drama and stalkery.
Just start thinking before you do things, tell him you love him, and let things go from there.
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my lastfm ^^