I'm not even sure where to begin, it sounds like you are dealing with a lot of different issues.
Do you guys have a budget? Written out on paper (or on the computer!) with realistic expenses? I think this could help you a ton, because it sounds like you guys aren't sure how much you have and how much you need and how much you can spend. It also sounds like your DH working solely on commission is stressful for you, so you need to find a way to manage that - whether that's you or DH finding a more predictable source of income (either instead of or in addition to his current job) or building up a large backup fund for when he's not bringing in enough money...
I do think it's true that no matter how much you have, it's easy to think you don't have enough... especially with the lifestyles most people in the US live. The basic necessities are food, water, weather-appropriate clothing, shelter (which includes heat in the winter)... but then if you have enough money for those things, it starts to seem like other things are 'necessities' -- internet, cable TV, cell phone, car, etc. If you have enough for THOSE things, then occasional vacations, or an iphone, or fashionable clothes, or music classes for the kids, all start to look like necessities as well. Does that make sense??? I think it's really easy to get caught up in that and forget about the basics & you wind up feeling like you never have enough.
As far as tithing... I started tithing when I was a kid. I didn't fully grasp the concept until I was a teenager, and then I became more committed to it, though there were short periods of time in college where I stopped tithing (because the Mass I was going to did not take up a collection & I didn't think to give elsewhere, it didn't seem important at the time). When I wasn't tithing, it's true that I didn't feel like I had enough money. When I started up again, I felt much more at peace with my financial situation. For me, there is something about the act of giving that takes the stress out of finances. I fully believe that God will provide & that has proven to be true in my own life. So tithing is very important to me, in ways that I can't fully explain. Because we tithe, we can't go out to dinner or movies. We can't sign DS up for that gymnastics class I was looking into. We don't have cable & would not have internet if my job didn't pay for it, nor do we buy clothes or pricey produce. But I don't feel like we are sacrificing, because I want to give, and giving is a priority, and we feel very blessed to be able to give so much. I don't really know how to explain it & I wish I could. But we are giving because we WANT to, not because anyone is making us. I think forced giving will likely lead to a lot of anxiety & resentment... But if you are interested in tithing (but just scared), you can do it... but I suggest don't jump right in to 10%. It's not like 10% is meaningful & everything else doesn't count -- any giving 'counts' and there is a story in the Bible about a woman giving just two coins because that was all she had to give -- and that was more meaningful than a rich man giving his 10%... (Mark 12:41) You can give only $1. You can give only 1%. You can aim to eventually give 10%, but if you find you still need that other 9% to feed your children, you can find other ways to give... give your time or skills, donate things you don't need, etc. But whatever you chose to do, give joyfully and freely, not with somebody forcing you... And there's a big difference between giving only $1 because otherwise your family won't eat, and giving only $1 because otherwise your family can't go to Aruba for a week, you know?
ETA: You might post over on the Finances & Frugality forum about budgeting and living on commission etc., I'm sure you'll get some great tips!
Co-sleeping is really wonderful when your child actually SLEEPS!!