S.E.
Start with a trip to the doctor. You would have to have a prescription for an anti-depressant anyway. They will evaluate your health and hormone levels first. Even if you do get a 'script for antidepressants, you will be required to see a therapist for a while to see if it helps. The meds only work if what needs fixing is medical and the doctor is the one to evaluate that. If that the issue, then they can be lifesavers, literally
Non-medicated help:
Try a weekday mass if Sundays are out of the question.
Get to know the other mothers from the preschool - you may be able to trade babysitting with one of them once a week.
Go to the library. It's calm, free, and books can be the 30 min escape you need on busy day.
Claim an evening to yourself. Let your husband know that every Tuesday (or whenever) you have the night off. From 5:30 to 6 the next morning , the kids are his.
Know that this is common, most of us have gone through this stage and come out stronger and wiser.