Hiya
I fed my babies with shields for 5 months each, so it can be done. I second the micropore tape for when you are still learning - I put a strip on the top of the shield, which held it eough to get started. I also found I needed to tuck a muslin under/in my bra, as when babe came off, all the milk in the shield would run down and soak me!
I only fed in feeding rooms to begin with, as I needed quite a lot of equipment, but it did get easier - much - as things went on.
Persevere!
I kept trying without the shields with mixed success - with my babies, they just needed to get bigger before they could feed without them. It was a combination of tham and me and fast letdown that caused the difficulties, not just my flat nipples, as my nipples got quite big during a feed, but my babies still couldn't latch that well.
If you can, try to find MAM/NUK ones - they are best, and I found they did not inhibit my milk supply at all.
I fed till my babies self weaned - at about a year, and couldn't have done without my shields in the beginning. they are a terrible faff, but were a lifesaver for me.
Best of luck !