You're on a very low dose, so that's good that it's working. As to the question of long-term damage, nobody knows, so there is no answer. And just to clarify, you have no lack of serotonin -- nobody's ever shown a lack of serotonin in anyone. What an ssri such as Paxil does is prevent the normal breakdown of serotonin so it's used longer than would naturally be the case. I think your decision should be guided by researching the medication, realizing "the blues" post-partum isn't necessarily a reason to be on a med or staying on one, and that discussing your concerns with an understanding psychiatrist may help you decide. This is an individual decision for all of us.
As much as I wish I could give you the answer you're seeking, you really need to address this with your doctor, especially in light of the MS. We are not doctors on this side of the forum and you need real medical advice as opposed to mere "opinions," which is all we can offer you here.
I do know there are far more natural ways to get serotonin and that is something you can discuss with your docs. You may want to check out the MS Forum and see if the folks there might have some suggestions.
I wish you the very best
Peace
Greenlydia