I am so on the same page with you. It is hard. Being a FT mom in today's day and age is harder than working 90 hr weeks, in the corporate world. Been there and done that and have the experiences to compare. I think I figure out why.
I couldn't think of any job out there, other than mommy, where the demands on your physical, emotional and mental abilities are required 100% of the time, when you are caring for the kids, at 100% levels.
What I mean, working in corporate world, I used my mental capacity (thinking, analytical,problem solving, judgement, etc) the whole time I was working at nearly full capacity (some tired days, or some reading email days, not 100%). Maybe I was plugged in emotionally a little, bit, but far from 100%, and I was definitely not doing a whole lot physically (sitting most of the time in meetings, behind a desk, little walking).
With a child (even more so with 2) there is no down time, we are plugged in 100% from all 3 perspectives. We love our kids, so we always want to say the right things, do the right things = 100% mental usage. We are physically active, between cleaning, laundry, games, lifting kids, and moving toys from the entire house back to where they belong, we are as close to 100% active as we can be, considering the other demands on our body/mind. Of course emotionally, let's not forget, we are 100% using the emotions, from tending to pain cry, or sad cry or just want attention cry, we always do our best to accomodate them/care for them.
No wonder we feel drained and exhausted. So, yes. the house won't be in order, things will never stop coming and we will forefer feel tired. Think about it, we do the jobs of at least 9 people that I can think of, dry cleaners, maids, cooks, nannies, escorts, drivers, assistants/organizers, psychologists (for our family members, even some friends), personal shoppers. I am sure we can think of more, if we really set our minds to it. Is it a wonder we're pooped?!
By asking the question above, and by thinking this way proves that you care and that you are a great mom. Hang in there. The things that help me the most are keeping a schedule as much as possible (sometimes it's very challenging), and keeping routines as much as possible - bedtime routine being the most important. If you can keep the routines (can be as simple, as dinner, bath, book, prayer), then you can work up to everyone down by 8:30, which leaves 8:30-11, as mommy time. You can take bubble baths, or at least a warm shower, reading somehting you enjoy, working out (but watch out, because your adrenaline may shoot up and you may not be able to fall asleep easily after that), watching your favorite shows (tevo and recordings come in handy), or even cleanin in peace and quiet (it's amazing how good that can feel, when you are used to constantly be doing it in a rush.) If you are a morning person, go to bed early and get up before the kids and do stuff like that.
Hang in there and good luck.