How can I be more supportive?

My partner and I are very close, & have talked about marriage and owning a home together. We have lived together for almost two years, and he helps me raise my son, who comes to think of him in many ways as his dad. Trouble is, I am finding I am not being as supportive of his endeavors as I would like. For instance, he has decided to go back to school after about 10 years, and it at first challenges me. My ex-husband and I broke apart after we were both attending school. We have had our own difficulties which we are also working through. Also I am trying to deal with my mother having terminal cancer with less than a year life expectancy, and some other stressful events. He has just told me about a chance to go river-rafting for a week (we are both very active) for free, and my first reaction is feeling upset. I am happy for him in both of these instances, and truly want to support him, but it seems to elude me at the moments when I need it. Somedays I feel my world falling apart.