how do you forget a name?

Maybe she does not think about your husband at all when she does not see him so does not use his name apart from the 3-4 times a year but is used to the other pronunciation.Letting it bother you is making you bitter and not affecting anyone else so I would advise stop dwelling on it, in the great scheme of things does it really matter.

Oh, invisiblegran, I AM that person! I don't mean I'm your DiL, but I have done exactly that to other people! Two lots of people over the years.
It makes me quite cross, but I don't usually apologise, as I don't want to draw attention to it. I am used to working with lots of people with a variety of names, so really should be more aware, but they just caught me out.

I then got so confused (let's take the example of saying 'zed' or 'zee' to keep it simple and anonymous).
I would say to myself 'This is zed, oh I usually make a mistake and say zee, or is it the other way round?'

Yes, it is disrespectful, and I don't know what to say to excuse it. I always just hoped that everyone knew I was normally a pleasant and kind person, and would forgive me. Strangely I was talking about it with my DiL this weekend, as someone at work is muddling her name with someone else's.

I have difficulty telling left from right and it really felt like that - a complete blind spot.

I'd also add that if your DH has a name that can be pronounced differently, he may be used to this happening occasionally (like Sara / Sarah) and that's why he's less bothered than you. I also guess that this is especially bothering you as you don't have a good relationship anyway. But I'd certainly try to park this bit of it.

Easily done once a name gets into your head you remember that ( probably the wrong version) or you get flustered and think is it this or that and just like buttered toast you get bet it ll be the wrong one
Don’t be too hard on her if that’s all she does and your husband doesn’t mind

I'm terrible with names and never called my in-laws by their names ever - they were just Mam and Dad. If your husband doesn't mind it doesn't really matter but it seems odd your DIL can't get his name right.

Because I don't work in an office I only used to meet my colleagues once a year at the annual meeting - it was a nightmare as by the following year I'd forgotten what they looked like again. We did finally get name badges but it was particularly bad when several new people joined at the same time as I used to get them mixed up!

I used to do weekly client work and the people used to wait in a small waiting area because I saw lots of people and sometimes only a couple of times I was dreadful with their names so I got into the habit if opening the waiting area door and calling the name before I actually went in the room so the person would be on their feet by the time I entered the room ( clever me) one day I did just that called out Kevin and up jumps this man we went into my room with me thinking oh dear I don’t remember anything about this man We started the consultation with me still madly trying to remember what he was all about and literally winging it suddenly there was a little tap on my door and one of my colleagues said I think you ve got my client with you
Red face me

Simply use the wrong pronunciation of her name whenever you see her.

Sauce for the goose is sauce for the gander.

It's easy. My best friend's grandson has a very similar name to my DGS and sometimes I call one by the other's name. My parents used to call my sons by a combination of both their names as they got confused. I constantly call one GD by the other's name and vice versa (not sure how you spell that either).
I love all my family but my brain, mind works in a funny way and I get confused. Fortunately I have a family who just love me as I am and it doesn't cause me a problem.

Oh doodle I m just the same, my grandson who I see the most is always getting called by my son (his uncle) name and my granddaughter is always getting called by my youngest daughter ( her aunts) name and sometimes I go though three of four combinations to get to the right one. They always pretend to be cross but they know me and know it is by accident
I really can’t believe you can get so upset about it I m sure it’s not meant to be nasty she’s just gets muddled as your son says ko

The brain is very odd. I do this kind of thing and not because I'm 67 - I've done it for a long time since 30s maybe. Don't worry about it. And if it's done deliberately take a deep breath and ignore ?