Lord I hate that "Smile, I command thee" thing, it drives me up the wall, but what can you say without being rude, except something like 'well, my (insert family member) just died" or something, and make them feel stupid. Of course you feel bad, it's your boyfriend's birthday tomorrow and it's great that you recognize what you are feeling and why, there really is nothing better than that kind of self-awareness, which I never had until recovery, and some days still don't have. If all else fails I just write out my feelings, swearing and barking and whining on the page until I feel better. It always seems to help shift my burden out of me at least a little. Hope it eases soon.