Honestly, some things like this are solved with learning certain skills, practicing them a bit, and then it becoming easier. Answering to "How are you doing?" does not require masking, people asking it in casual contexts usually do not expect any kind of "honest answer". It is as hard as answering "hi" to "hi". There are plenty neutral answers that neither require you to smile and play it happy, not invite some long, awkward conversation. It could also depend on where you live though, of course.