If the OM became single around Christmas and you found that the crush was deepening 'at the beginning of the year' it seems like an obvious possibility that what changed between you was the signals you were both emitting, together with a changed internal dialogue on your part. He was emitting 'available' signals (not necessarily to you personally and exclusively) and you were responding, together with an internal dialogue that perhaps this was more permissible, because you weren't now intruding on someone else's relationship. At the same time, his availability ceased to make this as 'safe' as it once was and this explains your discomfort.
People often baulk at the term 'it could be anyone' and I understand why that is, because it feels like I'm devaluing something that has become quite precious to you, while simultaneously giving you problems. Even a bad secret habit can become precious to us, because it's a part of us that no-one knows. Because this has been going on for so long - 2.5 years - it's become part of you that might be difficult to give up.
The other problem with the 'it could be anyone' suggestion is that there is a received inference that this is a shallow thing felt by a shallow person. It's neither of those things at all. All it means is that this person has come to represent something to you that's not personal to him or his individual qualities, but the feelings he stirs up in you, possibly in an area of yourself that has been untapped for some time.
This doesn't mean that your marriage or husband is lacking in any way. It would be impossible for him to replicate the headiness of the early days of a relationship, just as you wouldn't be able to compete with another woman who made him yearn for those initial feelings.
You emphasised in your initial posts that the 'spark' has gone a bit and you've rationalised that as being due to your relationships longevity. I admire you for your pragmatism about this, but it seems to me that however rational you are about that, there is still a yearning for the early days of sparks flying, intense and sometimes uncomfortable feelings and the pleasure and pain of infatuation. Perhaps you experienced that in a safe way until Christmas, but then his availability notched it up.
You've been married 15 years and you might believe that it's impossible to recreate those heady feelings after all this time. But the strange thing about a marriage crisis that is jointly experienced and tackled is that it can stir things up so much that complacency is stripped away and the 'lack of safety' this engenders revitalises the marriage and the feelings for one another. Numerous long-term couples talk about 'falling in love again' after something has happened that threatened their safety, either as individuals or their union itself.
I can't advise you either way about discussing this with your husband. You mentioned that you were previously close and told eachother everything and I'd imagine this has put a distance between you for what is now a long time. Only you know whether he will understand this, or will instantly mistrust you as in the case of a recent poster's experience. If you do decide to share this, you also need to be prepared to hear about your husband's own crushes and temptations. A conversation like this is a risk and requires a level of maturity and pragmatism not everyone has. When it works though, it can blow the cobwebs away and marks a more honest, grown-up phase in a couple relationship.
I think given that your husband sounds like he is suffering and worrying, it's not an option to do nothing. I think you need to be proactive about this and tackle it.