A relationship (romantic and otherwise) evolves and changes over time. Life brings circumstances that shake it up, turn it inside-out, upside down, wrong side up - and it is never the same. Depending on how you handle it, it can either cause a rift or a renewal. It is wise not to resist or struggle against it. Instead, try acceptance and surrender. Take a step back and several deep breaths, if you must. When you speak, speak gently and out of love.
When the chemistry changes, take the new form and examine it. Peel at the layers and beneath it, you will realize that your attachment to what once was (or what you thought it could be) is borne from fear of realizing the truth - about yourself, about the other person and who you are in relation to that person. It's best to take it as it is, and handle it kindly.
When relationships are rocked by some inner turmoil or an external deluge, just keep steady and weather the storm. Turn to your Anchor -that unshakeable, unchanging and unshifting force of Unconditional Love - and know that you are loved and worthy as you are - whether the relationship sinks or survives.