You're correct. 幸福，快乐 both have the meaning "happy". 快乐is a generic description, but 幸福 is usually to describe love, a marriage or family. 幸福also imply "satisfaction about life"
她有一个幸福的家。 She has a happy family.
家人的关怀，我觉得很幸福。 Because of the love and care from my family members, I feel so happy.
我有一个快乐的回忆。I have a good (happy) memory.
you can always replace 快乐 with more specific 幸福 to indicate it's love/family related.
放松 is apposite with 紧张. ( relax vs nervous )
辛苦 means feeling hard or painful ( either mental or physical )
他的工作很辛苦 His work is tough.
他生病了，很幸苦 ( He got sick, and he's very painful )
难过means feeling sad about something.
舒服 = feeling good / relax / comfortable
不舒服 = uncomfortable / feeling ill ( either mental or physical )