Could be any number of things really. Did you have a libido until recently or has it always been that one or twice a month was plenty? Medications, Wellbutrin shouldn't cause a drop in libido, but I don't know about Abilify. Past traumatic experiences, perimenopause ... or worrying about it.

I would say if your libido has dropped recently you should talk to your doctor, but it could also be that you just naturally have a lower libido.

First and foremost, I think you need to be sure you're healthy and nothing is amiss. Then from the relationship perspective, I like the Dr. Phil 100/100 view. Each person, your fiance and you need to give 100% to the relationship. This means that you both need to understand each other's needs and do what you can to fill those needs. If he as a once-a-week guy and you're a once every 2 weeks girl, find that middle ground where he is satisfied, but you aren't pressured.

Also, there will be times where you are tired and just not in the mood, but for him you could have sex regardless (and might find that you get into the mood and really enjoy yourself). At other times, he'll have to wait a while. It's all about you both working together on strengthening the relationship.

Edit: Oh! And normal is what you are so don't stress comparing your libido to someone else. Just be aware of changes, not the normal baseline for you.