I'll pass-on something that was taught to me over 25 years ago. It's the CEO's first rule: "The people you help the most will fuck you the hardest". And that's not a free pass to be jaded or to not help people either -- you still have to do it. Just be ready... because the more effort you expend to fix things or make things right with someone, the more entitled they become.
You see it at work all the time, so let me describe it there then translate that into relationships. The employee you make lots of accommodation for and show the most flexibility with will be the person who huddles-up five other employees and says: "I hate this fucking place". On the other hand, the person who shows-up every day on-time for 20 years and never wants anything other than the occasional pat on the pack will get overlooked. Its human nature to chase-after the lamb that isn't in the fold (remember Jesus' parable about the lost lamb?) -- but you have to train your brain to work differently. You've got to be able to let the malcontent go -- either passively or just fire their ass. Strangely enough, as unhappy as the malcontent will be, they just don't seem to move on. Instead, you have to retrain your thinking to go to the content, 20-year veteran and find ways to reward and energize them instead. The funny thing is, the other 60% of your employees who are between the two extremes will watch you help the loyal veteran and say: "I want to be treated like that" -- whereas before they watched you chase after the malcontent trying to accommodate them and said: "well I guess I should make noise too since he only helps the ones who complain".
Now relationships are different, right? Well yes and no. Find a partner who is fundamentally happy and appreciative. If you find yourself spending inordinate amounts of time and energy trying to make her "happy", then start wondering if all you are doing is trying to appease a malcontent. Instead of chasing someone else in a futile attempt to "make them" happy, find a happy person and enjoy their company. If you follow that approach, you'll spend your time enjoying the company of that person instead of pretty-much working for them. Because if you do spend all your time working-for them, they'll eventually fuck you over -- so remember the rule: "The ones you do the most for will fuck you the hardest".
Peace out!
P.S. Dump cancer girl as a girlfriend. Make sure she knows that if she ever needs anything (i.e. trip to the doctor, or some other closed-ended favor) that you'd be glad to help -- and do help. But end any romantic ties to her.