Unless she's in danger of seriously hurting herself, I wouldn't stop cats, chooks and whatever else getting in. She has to get desensitised to these things. As a prey animal, she will - in the wild, if they didn't become desensitised to "scary" things, they would die because they'd be so busy spooking at everything, they wouldn't have time to eat, sleep or mate.
I think it may be a problem with having one cow - they are natural herd animals and I think they take confidence from each other.
If you are feeding her, stay by the feed until she comes to eat, then rub her shoulder. If you're not confident getting too close, get a short pole that can act as an extension of your arm, so you can keep out of harm's way. She might move away but she'll come back to the feed - at least ours always did - and rub again.
Can she see out? When ours were housed, EVERYONE was sent to talk to them over the gate; everytime we went past, we called out to them, we stopped to talk to them, we played the radio (Classic FM was good, but Radio 5 Live has more voices).
Good luck!