You skim over the top part so quickly, it's hard to gauge how important having someone with shared interests, a family you get along with, a good sense of humor, and personality that meshes with yours is to your life. Even if I was cold blooded about the history you share (those eight years are "sunk costs" in econ terms), those things would be HUGE for me. But maybe they aren't for you.
Getting to your long points:
- Sexual lulls happen. Find something to spice it up. If getting head is important to you (and after eight years of not getting it, I find it hard to believe it suddenly is) then be more open about it. Be more open about what you like and dislike sexually, then find out what would really turn her on and do that. If all that fails, talk to sex therapist who could help you through it.
- Asking someone to commit to supporting two members of your family long term is a big ask. Not getting the "sense of inclusiveness and acceptance" that you want sounds like subjective hindsight now that things aren't going well. She said she would be okay with it once it became necessary. You pushing it further or not taking that as accepting what you want is on you, not her.
Also, are you two planning on having kids if you did get married? If so, did you think that might be weighing on her mind when making the decision to accept this premise?
- The "think of herself in times of panic"...like, those two instances are the best you can come up with of her being selfish in eight years of a relationship? That is absolutely nothing. Stop being petty.
- If you think that you're disappointed with her weight gain, think about how she feels. Not everybody is great at taking care of themselves or, after promising to do something about it, sticking with it. And I'm sure she's real keen on you "politely asking" about it.
If it is honestly killing the relationship, then figure out a way to participate in getting healthier together, as many people have said already.
Overall, you sound pretty selfish yourself and are projecting some of that onto her. If you are thinking of making the next step (either proposing or leaving her), you should do some couples therapy to work out these issues before you do something drastic. Make the time and effort to make the relationship work, because those things at the top aren't just window dressing, they are seriously important.