I think I know what you're going through.
How long have you been with your current firm? If it's been a while (e.g. a few years), and you've worked on other projects and with other people and it is just this particular assignment that is stressing you out, then hang in there. Bear with it and in a few months, when you're on a new project, things should ease off again.
However, if you're new to the firm, or it seems that it's some kind of underlying cultural problem where stress and long hours are the norm, then look for another job and move on when you find the right alternative. Life is too short to waste dealing with high levels of stress and long hours for people who don't give a damn about you.
Incidentally, if they're making you work ridiculous hours or calling you in at weekends, then stand your ground - if it looks like it's going to continue for some time. If you feel uncomfortable asserting yourself, make up an excuse (e.g. wedding to go to). It sounds to me like the project is under-resourced. If you're worried that standing your ground will damage your career prospects within the firm, and they really are making unreasonable demands on you, then I wouldn't worry about it - moving on might be the best decision you ever make.