Hi OP.
First of all, don't feel shit about yourself- it's really hard to keep 2 preschool kids entertained all the time, especially when you've got one that's younger than 4 as they tend to want you to play with them all the time and they can get in a lot of trouble if left unsupervised!
Ours were also having too much screentime and we found when we cut down there was some short-term bad behaviour, especially from the younger one (tantrums etc) but in the long term they are much nicer kids for it.
The first thing I'd suggest is model good behaviour- don't use your phone in front of them too much and not when you're supposed to be doing something else, like playing with them. Definitely don't use it to fill up time on public transport etc- talk to the kids instead, or play a game with them.
Do you allow the kids to use screens when there's "boring time" out of the house, eg on the bus/ waiting at bus stops, at restaurants/ in the car? If so, they're good places to cut screen time- these are times when you can chat and play games with them. They might get a bit bored sometimes but I figure boredom sparks imagination.
As for home, I found allocating a certain time for TV/ iPad (usually in the morning so I can get a bit of a lie-in or in the afternoon when the kids are tired) to be helpful. To make sure the kids stick to it I set an egg timer and told them that when the timer was up the iPad went away or the TV went off. I also don't let them watch YouTube unless I am supervising directly due to dodgy stuff in the autoplay/ autoplay being very addictive. Supervising YouTube is boring so it really put me off letting them go on it!
As for the rest of the time- I planned my day in the morning in rough chunks to make sure we stayed busy and weren't resorting to too much screen time. So, a typical day went something like this:
6:30 Breakfast at table, then some TV afterwards
7:30 Give the kids something to do (eg Duplo), play with them for a bit and then sneak off for a shower
8:00 Get the kids dressed, teeth brushed etc
8:15 Do some crafts
9:00 Walk to a playgroup
11:30 Walk back from playgroup
12:00 Make lunch- get kids to help make it
12:30 Do some baking
1:30 Go to the park/ supermarket/ play dates
3:30 Home, have a snack, watch some TV/ have some tablet time
4:30 Play with kids/ get kids to play with each other, read some stories
17:00 make dinner
18:00 have dinner, then bath and bed for 7.
If you treat screen time as a treat, you can use it's removal as a consequence for shit behavior. This was a great way to stop the tantrums in mine: "The TV is going off when the bell rings. Any moaning about it and there'll be no TV tomorrow". And then always follow through, it worked a treat.
Good luck! It does get easier- my youngest is now 3.5 and plays really nicely with his toys and sister- they're both on a screen time ban at the moment and don't seem to miss it!