what shall i do with those airlines?? shall i call complain? i am going mad here!
Which airline(s) and to and from where? The problem is that unless you are a frequent traveller with the airline, you are probablly not going to represent too much of a value to them and they are most likely to shrug your shoulders. Your travel agent should have been and should be your advocate in this situation, since their travel agency would have the most weight with the airline. They also have a financial stake in the game by getting paid by the airlines to actually book your ticket.
Schedule changes are a slight risk when booking early, but I am really surprised that you didn't get notified of it until about two weeks prior to the flights. I know a 12 hour lay over sucks, but it might give you an opportunity for a quick trip into a city centre somewhere and enjoy something you might not have planned.
In most cases, if there are other flights to the same place on the same airline during your 12 hour lay over, even if the flights are booked full, you can attempt to fly standby for no charge. If not everyone shows up for the flight, then the airline will give you a seat on the flight. This is a fairly typical thing and usually people earlier in the day often miss their flights but by the end of the day, there is ussually a fair amount of stand by passengers.
All in all, take a deep breath and realize that a lot of people are going to be dealing with some sort of similiar situation. Personally I travel by plane nearly every week for work and would have killed myself by now if I had let the stress of travel get to me. Especially during the summer, when the flights are fully booked, can get really stressful for everyone.