The thing is if you ve done nothing to it (like bleach, colour it etc) then there is a reason why it’s gone poor in four months As we are all in and not out and about in polluted areas, not doing it ‘poshly’ with hairspray, straighteners for work or public view every day, then I would expect it to be in better condition in four months, than breaking off and looking dry and dull, that’s why I think it’s really important to send the photos to your gp and ask his opinion and hopefully have some tests done.
When other things are ruled out then use all the tips you ve been given on here but I don’t think they are your first port of call, they are a solution but not a reason .... rule your health out first as that’s surely the prime reason our hair starts to look unhappy
Good luck