TTPD Cardigan falling apart after 3 weeks :(

Loveeeed the cardigan, wore it four or five times, didn’t do anything crazy like walk through thorns or anything idk. Today I realized the sleeve seam is unraveling. I’m so sad, it was too expensive to just fall apart this quickly. I think it was probably just defective, because otherwise I thought the quality was good. If anyone has any tips on how to fix it please let me know! Already tried reaching out to customer service but since I’ve worn it they won’t replace, which is frustrating but I get it.