It gets better. I felt exactly the same when DS was between 3 and 5. Six was better and seven is a dream in comparison but I'd imagine when you have two, the younger one is pulling the older one down to their level.
It is hard, it is thankless, you DON'T have to enjoy every second. The grandparents do because it's novel and because they don't get to have them all the time, and, yep, because they can shut them up with sweets, which as a parent you don't have the luxury of doing unless you want to deal with the fallout.
One day I struggled so much to get DS to the childminder that the next door neighbour, who was walking down the street in the opposite direction said "Hello, how are you?" innocently and I started sobbing on her
poor woman! She was lovely, and completely understood, her boys being 1 and 3 years older than mine.
Just some things which helped me cope when I was struggling:
Every day try to think of one thing you've done with them which you enjoyed. Just one thing, not the whole day, not some enriching activity, just one thing even if it was a fleeting moment. For several weeks with DS my bit that I enjoyed was the same every day - kicking a conker together on the way home. I used to worry that it mattered that there was only one thing I liked doing with him. Some days, the bit I enjoyed was bedtime, which at the time made me feel shit. I couldn't even find one thing in the whole day to enjoy. But now when I look back I'm glad that I took the time to think of what I enjoyed, because I now have lovely memories of those bedtimes, the special cuddle routine he used to want to do, and honestly I don't remember the hard bits. I know that it was hard, I know that I struggled, I know that I seriously considered walking out and getting on a train to nowhere in particular, but what I remember is those small things that I took the time to focus on at the end of every day.
Looking at them through the lens of a camera can help. You can literally edit out the bad bits and make it perfect which is soothing (I found it so, anyway). This also reminds you that everyone else's photos on facebook where they are having a great time - also the pick of the bunch.
Stop feeling guilty about screen time. Screen time is fine. DC need computer literacy for the future. It's really okay to use it to give yourself a break, too. Not everything has to be enriching and educational all the time. You do need to look after your own needs before you can look after other people's. It's also perfectly fine for you to have a break, you deserve it!
Ask your GP to refer you for blood tests to check if you're deficient in anything and your thyroid is working properly. These things can become depleted after having children. If you really can't/don't want to, get a multivitamin, or some vitamin D.
(Also, read this)
Reading hilarious blogs where parents admit to being terrible, so it made me feel better: www.renegademothering.com/2011/05/15/wait-i%E2%80%99m-supposed-to-play-with-my-kids/
One last thing one of my friends said to me - set yourself up to win. Don't try to have perfect days all the time where you're trying new things and it's all going to go amazingly. Just stick to things which are tried and tested and you know they will go for without a fuss. Kind of the pared down version of grandparents bribing with sweets, just let things go if they aren't of high importance, especially as you know you'll find things easier in a few months when school starts. It's NOT that important that they eat a fully balanced and varied diet, it's important that you offer them something to eat. It's not that important that they spend all day doing educational activities or even have under X amount of screen time, it's important that they spend a decent proportion of the day not annoying you or each other. It's not that important to validate every wish and emotion and stray thought that they have if a straightforward behaviourist discipline plan with rewards and penalties (pasta jar?) is more effective (I fell down on this one!) If that all makes sense? Just go for the easy wins, even if it's not what you want or what you think a "good mother" does. When you're feeling less depleted, then you can make adjustments and changes, but it's really okay to operate on bare minimum especially when you're feeling strained.
Hope things improve for you soon. The better weather should, also, help.