Both my parents have dementia. I have read, studied, and followed everything there is but still can't get them to take a shower or change clothes.

My Husband went through the would not change clothes as well. Luckily he never had a problem with showering until later stages.
He would take off his soiled clothes put on his PJ's and take his soiled clothes and fold them and place them near his chair and go to bed. Once he was in bed I would take the soiled clothes and replace them with clean clothes. If he had kept his underwear on I would grab them in the morning while he was in the shower and put a clean pair in their place. I don't think he ever noticed.
I never made a big deal about it was never worth fighting about something that could be remedied easily.
The showering thing ...
There are several reasons possibly why a person does not want to shower.
1. There are a lot of steps to taking a shower and it can get confusing.
2. It can be very noisy with the water splashing, the echoing of the water.
3. The head and torso are vulnerable areas and to have water hitting in that area can be frightening.
4. There is the possibility of slipping and falling.

the slipping and falling I solved by buying older used walkers at resale shops and my husband would hold onto the walker in the shower. We do have grab bars but he never got the hang of holding on to them .

The water hitting the torso area I solved by starting at his feet and wetting the feet first then working up. I also would give him control of the handheld shower. (by this time he was in a shower wheelchair.) A shower bench or chair would do the same thing. And I always gave him an extra wash cloth so he could help. He never seemed to be concerned about covering up but guys are a bit different like that ;)

The extra noise I solved by shutting off the water when I could, get wet, turn off the water, soap up, turn on the water...And I talked to him in a very quiet voice since everything seemed louder in the bathroom.

If a person has a real aversion to showering there are aids that can help and since they do this all the time they can sometimes get cooperation when it might be impossible otherwise.

And bottom line is older people do not need showers daily. Some of the oils on the skin do protect it and you do not want the skin drying out. As long as the "peri area" is cleaned after toileting that is most important on a routine basis. A shower 2 or 3 times a week should be sufficient. I was surprised when my Husband was in rehab I found out the state of Illinois requires showering or bathing only 2 times a week.

In Georgia we opted for hospice care for both parents. This is not end of life care, but more like palliative care. A nurses aide comes twice a week and baths my mom. She was great about making mom feel comfortable with her, and now our hygiene-resistant mother is used to it and has no problem.
it sounds like you could use some help. Have you contacted hospice for your parents? It has been a life saver for us!

Am I deaf, dumb and blind. What am I missing? There comes a point in time that you must accept no matter what you do to do the right thing, it is NOT going to be possible. What will happen? Well, eventually you will have major depression and the process of YOUR DESTRUCTION STARTS. When you simply cannot do the right and proper things and nothing on this earth makes that possible, it is time to ask yourself if things are working. If not, THEN MAKE UP YOUR MIND TO DO SOMETHING TO FIX THE PROBLEM - NOW. Sometimes we don't like what we will be forced to do but in the end, we really have no choices. If your "patients" are becoming overwhelmingly difficult and giving you endless problems, then you must put them somewhere where they will be looked after and cared for before they destroy YOU. You do not deserve that. I am sorry this is harsh but it is the truth. Sometimes we can literally kill ourselves for trying and only to find out nothing works. That is when we have to grow up and be strong and make some tough decisions. There is no other way.

I've had enormous problems getting both my husband and my father to have showers. My husband would joke that he would shower once a month, which was actually true, but he was really joking. He has shocking memory from his alcoholic dementia. He would often also recite his experience in Vietnam where he didn't shower for a month, when I asked him to shower. He used to be extremely hygienic. This problem was disgusting, we couldn't be intimate. One time it took several days to get the stence off me after I caved in to his demands for intimacy. He would argue against me that he didn't need a shower, he would think that he'd had a shower the day before, but it sometimes had been weeks. Both my husband and father will generally shower when asked to do this "the right way" by a professional Care Worker. Regarding my father, I surmise that the problem in me getting his cooperation is that I am merely his daughter. My father is 90yo and from a generation where your personal affairs are absolutely definitely not shared with your children at all. I believe he is hard wired to this type of thinking. For a long time he had terrible infected rashes, before I started assisting him, which I eventually discovered where just from bad hygiene. He just couldn't remember that he hadn't showed for weeks and couldn't work out which of his clothes where clean or dirty. He has poor eyesight and his sense of smell seems to be non existent. I even tried to get the ambulance to take him to the hospital to get him showered. They said they can't do anything and he can refuse to ever shower. I eventually started him with home care visits from a government funded care organisation and with the right Carer, we now get only just adequate hygiene. Some carers can't make him shower. Some can. The carers that "can't" get him to shower, keep asking him yes/no type questions like "would you like a shower". He will then of course say no, not wanting to inconvenience anyone. The only carers that can get him to shower, don't ask questions that require a yes or no response! They just let him know that "it's time for his shower now" sometimes encouraging him to do this before he gets taken out for a drive. He will be given the choice as to what clothes to wear, not whether to change his clothes or not, or whether to shower or not, Or he might be given the choice to either shower now or after breakfast for example. Now we don't get the infectious rashes and the stench that we used to get. One time my little boy caught the same infectious rash from my father, when my father would not where a shirt but was covered in open sores. As a result my son ended up in an isolation ward at the hospital with me, the doctors and nurses would suit up before entering to sort the rash on my son, until the hospital realised it was not an outbreak of a serious disease. They then just sent us home with some Cortisone Cream. I now have to remind the Carers that they must pass on information to me from my father, as my father confides in them. Most Carers just don't understand that he will not confide in me. This remains an ongoing challenge at times as most carers just don't understand that my father won't confide in me and that this is very natural behaviour. This is all part of my advocacy for my father. Sometime it feels like I'm constantly fighting for our rights and appropriate care.