I am hosting a family reunion June 24th for the day. How do I avoid feeling sad for no reason?

Often when I really want to be happy I feel sad for no reason. I notice little things of disappointment and can't get over them. For example at the reunion I fear my brain will get me down that it is not as much fun as I had hoped or that the majority couldn't come. I am in therapy and using the... show more Often when I really want to be happy I feel sad for no reason. I notice little things of disappointment and can't get over them. For example at the reunion I fear my brain will get me down that it is not as much fun as I had hoped or that the majority couldn't come. I am in therapy and using the doctor's ideas but just curious any additional ideas. It happens for example at Christmas every year. I have OCD Anxiety and some Bipolar.
Update: I saw the question above so I thought I would ask one last time as I am in a similar dilemna.