Here's some advice from a BTDT (my DD is 17 month older than my twin DSes).
Ladies, when you are beating yourself up over not "being everything to everyone", you are not only transferring that stress to your kids, you're also destroying your enjoyment of them in the here and now. You need to knock it off.
You need to do whatever it takes to knock it off. Maybe that means thearapy for you (this is one of the things I did). Maybe it means keeping a journal of 5 things that you liked about the day every day. Maybe it's time to do whatever you need to do to hire a housecleaner every other week (something else I did). Do Flylady or some other thing that reminds you that you're not perfect, never will be, and that is a good thing. STOP reading fancypants glossy ooooooh I am SO MUCH BETTER than you mommy blogs. Sometimes it means learning a little humility and forgiveness--for yourself (definitely me). But whatever you do, you need to figure out some way to be at peace with being a "good enough" rather than "the most perfectly perfect fantasy I always had for myself" mom.
My DD is now 9.5, my boys turn 8 next week. You know, we did not get out to library times very often (the boys went maybe once or twice); we didn't do mommy and me classes, the boys did do little gym butt didn't start until they were almost 3. We did do a once a week parent-and-child co-op group. Instead we went to (gated) parks, a little more during the summer when I hired a teen to go with us to the zoo, kids' museum, ect. We didn't even do t-ball or anything like that. They are happy, healthy, well-adjusted kids that are bonded to me and each other.
Once you've decided to dump the guilt over things that you really can't control and you're going to spend more time enjoying your kids rather than bemoaning how you're screwing up (and I know this is no easy task, but it's something you must do, and I know you CAN.)--then get in the habit of at least once a week, going out with each child individually (no matter how much the others protest). For a long time, for me that was having a grocery/errand helper. It's good for the singleton/oldest kid to not have to be overshadowed by the babies; good for each twin to not be totally lumped into a unit; and good for you to be able to shop without being stopped every third step by well meaning strangers (no matter how much you think you don't like the attention, you may discover that you kind of miss it--so it's good to help let go of that as well). Now that they're all older, I still do that, but also have a different kitchen helper each week, and now that they're all in different afterschool activities, I often get an overlap with one or two that allows me to spend one on one time. And they can always request a special time (like going out for tea or a walk) and I do my best to accomodate that. When you have a bunch of littles, though, it's catch as catch can. If you love them and are caring for them you're "doing right" by them. If you beat yourself up so they have a depressed mom who only looks at her failures, then you're not. Some people can figure out how to do that on their own, for others like me, it required some work in therapy for a variety of reasons. If that's what you need to, trust me, there is NO better investment of time and resources.
The first few years are really intense. But the skills you learn (acceptance of "good enough", self-forgiveness, compassion, humility, ect.) will serve you so well as time goes on, and you'll be ahead of the game with parents who never "had" to learn that until their kids are older.