my grandfather is cheating on my grandmother, he is now dating my girlfriend.

I am so sorry to hear about the difficult situation you are in. It is understandable that you are feeling hurt, betrayed, and confused by your grandfather's actions. Dealing with infidelity is never easy, and it can be especially challenging when it involves family members and those close to us. As a Buddhist, it's important to remember that all beings are subject to the same desires and temptations, and that we should try to approach this situation with compassion and understanding, even though it may be difficult. From a psychological perspective, it's important to recognize the impact that this situation may have on your mental health and emotional well-being. It's okay to seek support and help from friends, family, or a mental health professional to work through your feelings and develop coping strategies. When it comes to coaching advice, the most important thing is to prioritize your own well-being and that of your grandmother. It's important to communicate openly and honestly with your grandfather and girlfriend about how their actions have affected you and your family. While it may be difficult, it's important to express your feelings in a calm and respectful manner. You may also want to consider seeking the support of a therapist or mediator to facilitate this conversation. Ultimately, it's up to you to decide how to move forward and what kind of relationship, if any, you want to have with your grandfather and girlfriend going forward. Remember that you deserve to be treated with respect and kindness, and that you have the power to create healthy boundaries and make choices that are in your best interest.