Hi Erik! I am happy to help.
If she is continuing to treat you badly and tells you to "shelf your pain" in favor of her needs and bad treatment, that is not a good sign. When someone needs support while mourning a loss, they should not feel free to take out their issues on others. Getting upset, feeling sad or other related emotions is normal. But to expect you to ignore your needs and to accept poor treatment, along with being there for her to support her needs, is asking a lot.
It sounds like she is trying to use her father's passing as a reason to continue to treat you badly and it does not appear that she is not focused on the loss of her father as much as using that loss in to get what she wants from you.
Also, you don't want to go with your fiancee just because you feel guilty not going. Part of the indecision about going could be what she is telling you- that you need to support her and ignore your own needs and you feel guilty not doing so. The other part may just be that you are a normal person who understands that loved ones are to be supported during difficult times and by not complying with her request, you are doing something wrong. However, your fiancee is not willing to treat you with any respect or pay attention to your needs. And if you do go, ask yourself how it will help you in your relationship or change anything between you? Will she stop treating you badly? Are your feelings going to matter? Is this going to improve your relationship? If not, then guilt may be the only reason you are feeling the need to go.
You mentioned before that you are close to her children so if you can, you may want to explain to her children why you cannot go. Use age appropriate explanations and never put their mother in a bad light. You don't want to pull them into the emotional upheaval created by their mother. But letting them know you acknowledge their needs and telling them you care about them goes a long way in helping them deal with the situation.