How do you know if you're with the right person?

We've been together since school and it's been nine years since we got married. We met young, were inseparable right away, she talked marriage almost immediately and we've only been together in terms of physical intimacy. Mostly good times overall until about a year ago. I felt like we were one of those rare couples that bucked the modern trend toward separation and selfishness. But over the past year we've had some struggles so that I question things. One thing I don't understand is that during the past year when I was working hard, attentive to her, and spent a lot of time cleaning house, being romantic and so on (things I'd always done), she responded with little but contempt for me. She was open about her disdain and insulted me sometimes, though in ways that really hurt. She enjoyed spending time with other men, nothing physical but still somewhat disrespectful to our marriage. She told me openly on two occasions I wasn't her ideal man in not so many words. She made jokes about me physically and said openly that other men were more to her liking. I was not her type though we are married. I felt like I had put in a lot of effort for nothing, getting home from a long day, working around the house so she wouldn't have to, listening to her complaints, and was ignored or spoken to with contempt. If I failed to hear her complaints or talk with her, she got angry. But if I did listen and talk, she would end by criticizing me for not getting work done or cleaning up. Same on the romantic front, I would take her out several times a week, and she wouldn't care or would say she wanted something else like a better restaurant. People would say that I should take her out once a month and so on, but then when they heard I took her out all the time they were baffled. I basically decided I wouldn't try so hard, stopped doing so much housework, focused on my career, and basically told her if she needed someone to complain to she should tell a friend. Now over the past month she has done an about face and seems to work hard to keep me happy, fewer insults, apologies for the past. When I treated her well was I a second-rate man to her, but when I began to turn away and not try so hard, she changed her behavior and attitude. How can I have faith she's who I'm meant to be with?