I totally agree with what has been said already, and I could not have expressed it in a better way: You might be grieving for both at the same time, your dad and your mousie, and, very importantly, you felt so responsible for little mousie, and you had a very strong bond. So this is why it is so hard to get over it.
I have lost several pets, many rats, some piggies, some rabbits and many cats. It was hard each time, really, really hard.
But when I lost my Pauline last year, the piggy who needed constant care (syringe feeding, cleaning, many vet visits, a lot of research because nobody really knew what exactly was wrong with her), it was the hardest ever. Not because I did not love the other pets - I DID love them all (especially my ratties!), but because Pauline was on my thoughts 24/7; when I was not actually with her, I thought about her all the time, about ways how to help her, what else I could do for her, I wrote e-mails to specialists and other piggy mums (this was, by the way, why I joined this forum!), tried different vets, bought some nice things for her, to make her life more comfortable, slept next to her cage to see if she is alright - I was totally exhausted and tired, and of course more vulnerable than when you lose a piggy suddenly.
Moreover, I had to eventually make the decision to have her pts, which was the worst. It is ALWAYS bad to make such a decision, but in Pauline's case, I struggled so hard and finally had to accept that I could not help her any longer.
So the feelings of grief can indeed be very different, and you should not think "I am grieving more now, does that mean that I did not love my father as much" - but the feelings are rooted in many different "canals", I think, and there are several reasons why you seem to feel more devastated now than you did last year. Sometimes, when cases of death add up, it is also difficult to bear simply the addition of them; it is the same with other bad things that happen to you in life: you are often able to deal with one, but it is getting harder if you have to deal with one after the other.
Anyway, I can totally understand you, and I hope the three little girls bring sunshine into your life!