Hi OP, some of the comments here on this thread are pretty crazy!
I think you are doing pretty well, all things considered. You have gone from two incomes to one income. $100,000 for a teacher is pretty decent, but it isn't the household income you or your daughters were used to! It sounds like you are doing the best you can. Your mortgage is low, and you have a paid-off car.
Do you track spending? It's the best thing for people with a need to spend less. I use YNAB. There's a free 30 day trial and after that, it does cost money - annual subscription - but wow, it is worth every penny. Or, if you are in any way a student, you can get a one year subscription free with proof of enrollment (even just one class). And, if you have an account, you could create one for your kids as well (if they have their own bank accounts) for no extra cost. it's a fantastic way to track spending and make a budget. There are other apps out there - I know of Every Dollar, which is a different type of budget but also very effective. Or go old-fashioned - use a notebook to track spending, or a spreadsheet. Whatever works for you!
$100,000 income is not bad, but can you increase that at all? Will your school district offer higher pay for things like National Board Certification, or earning a Master's degree, or MA+30, MA+60 etc? Especially if they will pay for courses, you should be taking classes every semester to get into the highest salary scale. This is basically passive income. You have to do the work to take the classes, or get the NBCT, but once you do your salary will increase for the remainder of your working years. So if you haven't maximized that, look into doing so ASAP.
Your high schoolers will likely be looking forward to college soon. Do you have a plan for paying for that? I recommend the Facebook group Paying for College.
What do you teach? Probably the best way to supplement your income (aside from moving up the salary scale) is tutoring. The highest-paying specialties are math and things like AP prep. I think you should try to supplement your income by maybe a hundred dollars every week. If you are at the point where you are really feeling an increase of $50/month in your mortgage escrow, you would benefit from bringing in an extra $400 monthly.
Someone suggested mrmoneymustache and I think that's a great website (and associated forum) but you might not be ready for that. They have fantastic ideas for minimalistic living. If you can cut every category by 10% you may find you can live on your income instead of going into savings.
Years and years ago I read some great books called The Tightwad Gazette for ideas about how to live very frugally. The author shared one concept that stayed with me. It was the idea that you can reduce costs in three ways. 1) get the item more cheaply (on sale) 2) Make it last longer 3) Use less of the item.
So, with the price of coffee going up, how do you save money on coffee? 1) First, you can shop around and buy it more cheaply. (Buy it in bulk, with a coupon, buy a cheaper brand, get it for free somewhere) 2) Second, you can make your coffee that you buy last longer by stretching it. Use less to make a pot, brew it in a way that uses less coffee, etc. 3) Finally, just use less coffee. Instead of drinking 3 cups a day, drink 2 cups a day.
To save money on coffee, any one of these things will help. If you apply all of the strategies you can save even more. If you apply these principles to one item, like coffee, you might save $10 per month. But if you apply it to EVERYTHING you spend, you might save $10x40 things per month... instead of trying to earn $400 more.