headache, worry, vexation, concern - something or someone that causes anxiety; a source of unhappiness; "New York traffic is a constant concern"; "it's a major worry"

dead weight - an oppressive encumbrance

fardel - a burden (figuratively in the form of a bundle)

imposition - an uncalled-for burden; "he listened but resented the imposition"

pill - something unpleasant or offensive that must be tolerated or endured; "his competitor's success was a bitter pill to take"