We've all seen it, if we've been around poly folk over time: Apparently strong couple, with great relationship skills, probably even poly relationship experience, who are in love with each other. One of them finds a new partner, falls deeply in love, and goes over the edge into NRE/limerance, leading to the demise of the original couple. It's sad, it's tragic, it's regrettable, it's a mistake I have made, and, in my opinion, preventable.
Let's say you have planted a tree. You provide the tree with a certain amount of water, sunlight and food. You've been tending it for years. The tree has grown beautiful and strong, with branches that spread, roots that spread and dive deeply in the ground, glossy leaves that drink in the sunshine, and you feel pride and joy as you look at it, listening to the wind rustle through the leaves.
You find a lovely sapling, and decide that you want to plant it too. You dig a hole and set it next to your other tree. It's so different than your other tree, so very special. You pour yourself into learning how to best care for your new tree. Lavish it with fancy plant food, all the water it can drink, and the best spot in the sunshine. It's thriving! You feel happy and proud.
After a while you notice that the first tree is looking a little lackluster. The branches are drooping, and the leaves are starting to drop off. Thinking back, you realize it's been a while since you watered it, pruned it, gave it food, or rotated it into the sunshine. You stick a hose on it for a while, and work more on your new tree. The first tree isn't doing so well, but the new sapling is looking amazing.
You decide there must be something wrong with the first tree, and opt to let nature take its course. After all, if the roots are deep enough, surely it will survive on all the time and attention you've given it in the past.
Do not sacrifice the health of an existing relationship to the growth of another.
Certainly, there will be a reallocation of resources, and shifting of focus that happens when a key new relationship enters the picture. This doesn't mean your existing partner(s) should be left in a position where available resources do not sustain them. Nor does it mean your new partner just gets left to dangle while you attend to your previous connections. Intense NRE is difficult for most people to navigate. Finding that balance is a process that takes time, patience with all involved, and repetition to master. You will screw the pooch. Your older partners will. Your newer partner will. It's a given. Expect it, and form strategies for moving forward when you fall short.
Build the skill set of spending quality time and focus with your old partner(s), especially when the New Shiny is more appealing. It can be uncomfortable in the beginning. The imperative of desire will push you in a place where you want to spend all your time, attention and focus with your new partner. Your older partner knows that. Even when you're with them, there is often a lack of focus that is perceptible. Keep going. Learning to shift gears between partners is a survival skill in polyamory. It's ok to say, "I'm glad to be spending time with you, and I love you. I'm also missing New Shiny right now, and feel a little conflicted." then keep spending that time, focusing on the partner you are with.
I've heard people say that the organic growth of their new relationship isn't something that should be limited, slowed down, or restricted by their pre-existing partners. That's crap. If you leave the station wagon in the garage for months, and only drive the sports car because it's summer, and you feel cooler driving it, don't be surprised that the station wagon won't start up when the rains come. The battery dies. The spark goes away.
Make the changes you need to to grow your new relationships, but be mindful that you aren't cutting off your existing partner(s) to do so. If you are fortunate, even your new partner will want you to keep the rest of your relationship sphere happy and healthy, and be willing to miss you at times to make sure there is space for that to happen. Build your own personal forest.