It's not self-centred, it affects everyone and often those supporting the family can feel just as much pressure but feel that their stress is somehow less valid because it's not their actual parent. Whatever you are feeling is ok, it sounds like you are being absolutely wonderful and DH and mil are very lucky to have you.
There is nothing wrong with a bit of time out, absolutely nothing. If you need a break from it then do plan something with your dd one day.
You need to talk to your DH though, communication is so important when it comes to dealing with this situation. Tell him that you feel things have gone past the point where you can simply keep pretending everything is ok, and that you yourself are feeling the burden. It has to be addressed for your mil's sake more than anything. I can absolutely assure you that she will be well aware of what is happening to her, people in early stages of dementia usually are, but it's so hard for everyone to address. It may come as a relief to sit and talk through everything frankly together, it would be a huge step forwards in itself.
As for how you imagined family life, again I understand. In many ways this is very much why you (actually your DH and his DB) have to reach out for as much help as possible, you shouldn't be dealing with it by yourselves. However, I'm sure you of all people know that sometimes life is just cruel, I'm so sorry to hear about the loss of your father, and can imagine this brings back many feelings that you hoped you had put behind you when you went through that loss. When you need to get away, do. Don't feel guilty.
You need a plan to move forwards and communication has to be the first step.
The GP is the next, with a referral to the right team to assess your mil and determine the right help to you all as a family that might be available. Please stress any risk that you may believe she is to herself or others, either consciously or not (eg not being able to recognise dangers in the home) as this information is critically important in helping a very very over stretched service area prioritise their caseload.
Then, you need to explore every option and allow time to make the best decision.
Posting here is in itself a step forward, you have realised it's too much for you to support the whole family when no one else seems to be taking control of this horribly sad set of circumstances. You are all important, but there are sadly more cruel times ahead and how you deal with it now will directly impact how you all get through the future together.