Hi sunbeem... IMO your own welfare is most important right now. If you think maybe in the end it might be OK to spend the day with the invited neighbors--maybe you'll feel better being around people--try it... But if you feel it will bring out bad feelings and/or that the stress of trying to pretend will make you feel worse afterward then I say (again my opinion) that protecting yourself by staying away with a gentle excuse is a perfect alternative. I don't know the situation with your husband but I hope he would understand and want to do what is best for you. If you can, talk to him about it.

For the longer term I think it is important to see your T. I don't have much to add to the excellent observations and advice that Frankbtl offered. It is understandable the desire to cancel a session, try to get back to "that place" where you are able to go, and rely on us here to try to help get you back to that place.