Does anyone feel so sad after hanging out with your partner's/SO's awesome normal family?

I'm finding it more and more difficult to be around my partner's perfectly nice, very loving family. And I feel terrible around it. It's hard to explain, but being around them makes me really close up and shut down. I dread the time before it happens and then feel so sad after a visit, probably because it brings up all of the things I wish I had experienced growing up. I don't think I feel jealous as much as just a deep hurt and longing. Does anyone else relate to this at all, and if so – how do you cope? I am starting to want to make excuses to not do anything that involves them because of my issues. I know that's not right, so am trying to get myself back on track here.