I just wanted to say that you are absolutely justified in your feelings. What you are doing is heroic. You are living the gospel HEROICALLY. And that is never easy. And the Evil One wants to stop you. He wants to destroy the HEROIC example that you are setting for your children. Because if He can do that, there will be four more people in world who believe His lies instead of the TRUTH of Gospel.
There are many ways to avoid a pregnancy, without complete abstinence, that are approved by the Church. They will all take some time and effort on your part and on the part of your husband to learn, and feel confident in. I think that what you need right now is some practical help and advice.
I would recommend, that if you aren’t already sleeping with your newborn and nursing her through the night, you should be. If you aren’t, you may already be fertile, but it can’t hurt to sleep with her and it might keep your fertility from returning. That said, you can also start to chart all of your signs so that by cross referencing you can get a better picture of where you are. You should also make sure that you nap as much as possible, even if you have to hire a babysitter. I know that money is tight, but try asking around at church and you may find a lonely old woman who never sees her own grandchildren who might like to help. You should also do your best to eat as much fresh fruit and vegetables as possible. And your son, maybe after all the other kids have gone to bed, you could get him up and just sit with him and watch a video or read some stories, just with him. Nothing to profound, but I have found with my own children that sometimes a few minutes with mommys undivided attention can really help. And you might have to do that once a week.
My prayers are with you,