This situation can be challenging, as it involves the feelings of both your bestie and your girlfriend. First, it's important to have separate conversations with each of them. Talk to your bestie about her feelings and your decision to accept her proposal. Be honest about your emotions and the circumstances that led to your choice. With your girlfriend, explain the situation truthfully and express any remorse you may feel for the sudden change. Remember that open communication is vital in both cases. Respect their decisions and give them space if needed. Moving forward, prioritize clear communication and consider the impact of your actions on others' feelings.