It’s not so funny how you think that what you and another person have is solid but, it turns out to be quite fragile. What’s really going on here I asked myself (and God) until I received an answer. Make it make sense. The simple answer is you may have been way more invested in it, the friendship, relationship, partnership than they were. It may have been layered with somewhat equal exchanges but the EMOTIONAL investment for you was much greater. Therefore, you feel a bigger let down and may have difficulty “wrapping your head around” their ability to move on as if it was nothing. I’m not saying they didn’t care. I am saying it was just not as much as you did. The bond was only on your end and your “deep” connection was more than likely a one way thing.
~Nikki
Nikki's Confetti Life 
Married couples, even those married many years, can and do go through this as well. We have many times in our marriage when we are off kilter. Why not? We are people. I want things and Sheryl wants things or places, or experiences or the shoulder to cry on at different times.
How then do you get though it? We have to have shared overall aims. I want us to have financial stability, so do we. I want us to enjoy life, so do we. We want us to feel fulfilled, so do I.
Its not secret sauce but it is hard work. It is more work than two non married people need to do. Instead tow unmarried people should be investing in finding out if they have shared goals. If not, it’s best to move on down the highway.
rick