My boyfriend used to be so sweet and loving, but he changed?

He used to tell me how much he loved me, and always wanna spend time with me. He cried over me and ditched his friends for me. We've been together a year and a half now and we live together. now he goes out all day everyday to get high with his friends, and never wants to be in the same room as me. he sleeps on the couch watching tv all night, and when I ask to cuddle he says he doesn't want to. I just wanna know why he changed but he never has an answer for me. I have a feeling he's cheating because of some emails I found on his phone but he denies it. what are some signs he could be cheating?
My boyfriend used to be so sweet and loving, but he changed?
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