I'm glad Aitchbee came back with a positive report on the Lewy Body Society website and forum. I hope you can find some helpful information there.
I also found the American version:
https://www.lbda.org
I find the information on the UK's Alzheimer's Society website to be a great help. It's clearly written and usually thorough. The US Alzheimer's Association has similar information, but I don't think it's as well written. I imagine we've all spent sleepless hours Googling the depths of the Internet.
Again, I can't speak specifically to LBD, and know there are some differences with how it presents to Alzheimer's. I also know that every person with dementia is unique in their experience and presentation.
Having said that, there do seem to be common experiences that many of us have experienced in our person with dementia (PWD), or heard about here on TP, or in the local carers' cafe, or from others. (My PWD is my mother, age 73, diagnosed with Alzheimer's a year ago after a crisis and sectioning in hospital, probably moderately advanced, now in a care home near to me, and with no short-term memory as well as the dementia.)
Delusions (false, fixed beliefs) and hallucinations (experiencing sights/sounds/etc that are not there) of different types, are not uncommon. I am thinking that there may be some link between having LBD and having certain types of hallucinations, but am not sure (told you I am woefully ignorant). If the hallucinations are not upsetting, that's great, and I hope they stay that way.
It's possible there could be some depression going on but it can be hard to tell. I do know you have to be careful about antipsychotic, and other, medications with a patient with Lewy Body dementia but of course everybody may react differently.
One thing I know is common to caregivers of anyone with dementia, is the stress and upset and frustration. It's such a difficult, consuming disease to deal with and of course there is no cheery prognosis. It's degenerative and terminal and we know they will just get worse, and I can't think of any worse diagnosis.
I definitely know the "useless" feeling, because there isn't anything you can do to stop or fix the problem.
However, you are not useless. Your dad (and the rest of your family? is he at home with family?) will need help with all sorts of issues, and not just the hands-on caregiving. There is legal and financial stuff to sort out (I will leave it to the UK residents to advise you, but power of attorney and bill paying and money handling and navigating the system, for a start), there will be medical issues to oversee (doctor's appointments and medication and so forth), the issue of caregiving (getting carers in, finding a local day care program, carers' groups, activities for your dad like Singing for the Brain, et cetera), and practical things around the house. Do get the legal bits sorted as soon as possible but don't feel you have to do everything immediately. Someone told me that this is a marathon, not a sprint to the finish, and you have to pace yourself.
I spent (and still do, some days) a lot of time at the beginning reading past threads here on TP, to get an idea of what other people's experiences were and to have an idea of what I might be facing. That may or may not be helpful for you.
Something else that helped me was to find out about local support options in my area. I've been to a number of different support groups (carers' cafes or carers' groups) in my area and liked some better than others. I have also been to some classes and workshops and lectures. These have all been useful because: I learned something, I sat in a room with other people who understand, I got a referral for a solicitor, I made a contact, I found out about other support, et cetera. So I would encourage you to do that. You might start by ringing Age UK and the Alzheimer's Society, or maybe the UK residents here know better how you can access that network in your area.
I've learned a few things about dementia in the past year:
-if someone offers help, don't think about it, just accept!
-dementia will eat up as much time and energy as you can throw at it
-don't let it consume you; you and your life are important, too (easier said than done)
As far as feeling you are lying to your father when you tell him you will go on more days out when the weather is better: STOP feeling guilty and berating yourself (as best you can). If you let it, the guilt will eat you alive, every day, until there is nothing left.
You were spending time with your dad and reassuring him the best you could. Whether or not it's literally true has, to me, no bearing on the situation. You were being kind to your father and hoping for the best, and said what you said with the best of intentions. Kindness and hope are good things.
Nobody has a crystal ball to predict the future. You could have many outings with your dad to come, or you could get hit by an anvil falling from the sky as you read this. If you tell a friend you will have lunch next week, and then you don't because you get sick, did you lie to her? No, you gave her the best information you could at the time you made the statement. That is all any of us can do.
On the subject of lying, some of us have found that in dealing with dementia, it is often kinder not to be completely and openly truthful about everything all the time, and especially not helpful to volunteer truth. Someone here will post the compassionate communication link, but the idea is that rather than correct the person with dementia when they come out with something that isn't true (and possibly upset or shame or distress them), it's kinder to just go along with things. So when my mother sees a seagull and says, "look, it's an eagle!" I don't say "no, mother, it's a gull, don't be stupid, can't you tell?" but rather just go along for the ride. It does no harm, it doesn't matter what kind of bird it is, if I contradicted her or corrected her she would get upset, and when she gets upset, it's really not good for her. My goal is to keep her as calm and content and anxiety-free as possible. So who cares if it's not an eagle? If she is going on about one of her many delusions I just go along, try to validate her feelings, and try to reassure her, and then try to distract her. ("Really? Someone stole your socks? That's terrible. Tell me about that. I can see you're upset. Of course you're upset; anybody would be upset if their socks were stolen! I'm sorry you are so upset. I will look into the missing socks for you and let you know what I find out. Let's go get some ice cream." It sounds like something out of a bad textbook but if I can manage to get the body language and tone of voice correct, it can work, and then she doesn't spend hours being upset about socks.)
I'm not saying this is instinctive or easy to do, and it works for some of us and not all of us (and some of us only some of the time) and I often feel I am an actor in a strange play, or doing improv comedy or something. But I do think most of us have been there with the bizarre moments (and you can see the long-running So Bizarre! thread for many examples).
This is an awfully long-winded way to say, you are not alone, your feelings are reasonable and understandable, you are not a bad person, try not to beat yourself up, try to be kind to yourself, do what you need to do to take care of yourself, and remember that TP is always open when you need it.