When the initial spark is gone, what next?

To add on to this, it's not the responsibility of other people to make you not feel bored. A relationship may be there to enhance and share your life, but it's not to take all responsibility off you, fix your problems for you, or emotionally regulate or manage you. That's expecting too much from sources outside yourself. If you're feeling bored, are you generally content being on your own and sitting with emotions and life, or do you seek a lot of distraction through variety, stimulation, and novelty?

There's a difference between feeling bored sometimes but being able to find ways to get yourself out of it versus feeling numb and disconnected when you're not being highly stimulated (which I'd argue is what limerance feels like, highly stimulating). Life is always going to have tedious and boring parts, chores and work for example, and everyone needs to contend with that and make the best of it.

In regards to how attachment factors into this, I do agree with introvert that intense limerance usually reflects attachment wounds being heavily activated. There's a lot unknown about the new partner at the beginning that gets filled in with fantasy, plus hormones are doing all sorts of biological things that feel good. As you get to know the person and fill in the gaps with reality and you get used to them so the hormones calm down, the overwhelm inevitably fades (which believe it or not is good because limerance in the long term puts a lot of stress on the body). I've observed that with insecure attachers, the boredom usually comes from resuming being in your usual state, one that originally is filled with difficulty sitting with yourself, difficulty fully processing emotions, and (depending on your past history throughout your life and your genetics) feeling numbness or nothing or boredom if you're not triggered anxious (or even possibly avoidant, triggered in one direction or another -- but the baseline space between being triggered, when you're alone with yourself and have nothing to react to, feels stifling).

I know I just went super intellectually analytical about something that feels like it should just be emotional. But emotions are a guide, and one struggle for insecure attachers is figuring out what they're really trying to tell you. If you have a pattern over and over of losing attraction and connection for promising partners after a few weeks or months, then your emotions are correct in giving you information that something is wrong! But it's more likely internal issues that need to be addressed and not anything wrong with your partnership once this is a pattern... either because it's difficult for you to sit with yourself or because you're only attracted to unstable or unsuitable partners (I used to be AP and was only only only attracted to avoidants, and each pairing had the same disastrous dynamics over and over).

I find this link really informative in considering what is potentially going on underneath the kinds of dynamics you described: jebkinnisonforum.com/thread/2372/overcoupling-stress-response