but i honestly dont think i can do it
...and I don't think that is at all an unreasonable thing to say.
Some people can, some can't.
Even if you can for one person, that doesn't mean you could for another person with the same problems. For instance, I have cared for Jan for 15 years, but I don't think I could have done it for my own Mum so closely. Dreadful thing to say, but it's true. I'll never know of course, because it was never asked of me. Had it been asked, I don't think I could have asked Jan to share it with me. Ah, all those what if's...
Had I been asked if I could have done it for Jan, I'd have not have expected to have the internal strength. Amazing what one can find inside oneself.
The questions you need to ask yourself are numerous, one of which is - who would actually be doing the active caring?
Can you elaborate on what
mother in Laws dementia has got to the stage
means? How exactly is it adversely affecting her at the moment? What is the development that has made him suggest this? In such a situation, what would either of you be able to do?
At the current stage, I think you both need to look at the situation, review her condition, look at what options are open and what resources you have [time, ability, willingness, strength, etc] as well as acknowledging how long it might go on for [given her age and physical health]
Has your husband seen TP at all? Does he know what might be asked of him in the caring?