You are not a failure. Every mother has selfish feelings and that's a good thing. If you take care of your own needs you have more in your emotional reserves for others. If you really feel yu are about to lose it you need to take a little break. Make sure the children are safe and can't hurt themselves/each other, then go in another room and take a few minutes to calm yourself. My 2 youngest children are only 14 months apart. I remember those feelings well.