Just telling your boss you're under stress won't get you anywhere. If you want allowances made for stress, you have to offer something too. What are you going to do to manage your stress? Everyone screws up on a job sooner or later. One particular screw up doesn't usually cause an employer to want to dump you. What concerns the employer is if that screw up is part of a larger pattern. Even then, an employer can decide to be supportive of you, if you give acknowledgement that you have a performance problem and can come up with a plan to correct that. You can even ask your employer for advice on designing that plan. But you have to have some ideas of your own on how to better manage your responsibilities at work. A sincere commitment to doing so can very favorably impress a superviser. Your stress is your problem. Being kept on until we outlive our usefulness is pretty much the fate of anyone on any job. "It's not personal. It's business."

Your husband being out of work while you are awaiting a baby is a very tough circumstance. Add depression to that, and you certainly are coping with a lot. Your mom is in denial, thinking that there's some pill you could pop to offset all that. Getting an additional psych diagnosis isn't going to help much either. There probably are good psychological explanations for your performance deficit. You could sit down with an a analyst and explore all that. Might be interesting. Then what? Ultimately, it's on you to make yourself valuable to your employer. If you can't, then you don't belong on this job. Even that would not be the end of the world. It might mean you need to do something else where you could be more successful. But, first, try and make the most of the opportunity life has given you with this job. Put aside your understandable desire to be understood by your boss and coworkers. Try and figure out how to do better. Let others know that you are working on your problem. That's the best way to get support on the job.