At my wits end

I have spent a good deal of today crying. (I know that's not a cheery way to start a post, but It's true) It seems that when it rains it pours at my house and after an eventful day yesterday, with the dumping of pepper all over the kitchen, toilet water all over the bathroom and Styrofoam all over the house, today was just enough to make me feel like a failure of mom.

When Dyson was little he was really quick to learn. He spoke early, he learned his ABC's quickly, and he walked young. I knew the things he was doing were fairly advanced for his age and assumed that my other kids would be different and so I have tried not to compare them. The problem has come that Ridge has not only not learned things as quickly as Dyson did but, now that he is 4 he still doesn't know his letter names and sounds. He can sing the alphabet song but he has no idea what he is saying or what it means. I have tried flash cards to help him learn the letters and he can copy what I say when the card is right in front of him but as soon as I move on to another letter or put the cards away he forgets everything!
I don't really know what to do or what to make of it. Today we were trying to learn the letter L and I asked him what letter it was and what sound it made and we seriously worked on it for 30 min and at the end of it he still couldn't tell me what letter it was or what sound it made! We laid on my bed thinking of words that started with the letter L and I would say something like "lotion" and he would say "doorknob"!!! After 30 min of trying to learn the letter L and he still can't come up with a word that starts with the right letter sound.
I must be completely ineffective as a teacher because I have literally tried everything I can think of to help him learn it and he just doesn't get it.
I enrolled him in preschool when he was 3 so he could start getting some of the letters and numbers figured out, but I noticed that he really wasn't getting it. I chalked that up to him just being one of the youngest kids in the class and thought maybe he just wasn't interested. But now that he if 4 and is not any closer to figuring it out I'm not really sure what to do.
So after our little letter adventure I laid down on my bed and cried.
I feel like a failure as a mom and as the teacher to my children. I have to admit that I did lose my temper and that of course never helps. The light of learning hasn't clicked in him and I don't know what to do to turn on the lights.
So to sum up the day. I've not been feeling super great, the house is a mess, I lost my temper with my kids and I have spent a great deal of time crying.
Being a mom really is the hardest job in the world. Just as the paydays are the greatest so are the disappointments and failures. The fear of failing as a mom, especially a stay at home mom, is almost crushing to me. The question "What do you do all day?" is not an uncommon thing to hear out here and when there are few victories to report it is really discouraging.
I have always felt that my place was in the home and that I could do the best for my kids to be the one raising and teaching them and daycare and babysitters were not what I wanted for my kids, but I have to say that if you were to compare my kids to those who have attended day care they are lacking in the academic areas. So what do I do all day? and how can I be more effective in teaching? I have no idea and daycare is looking pretty tempting right now.
To add insult to injury in this whole mess, I want to put Ridge in preschool so he can hopefully start to understand it but I pretty much have no options for that. All the preschools I can find out here are daycares and super pricey (the one next door is about $1200.00 a month) and I haven't found anyone do to a co-op preschool with. So poor Ridge is left with his unqualified, impatient, discouraged mom who will be severely distracted come Dec. to teach him everything he needs to know before Kindergarten.