Alright, there are several different issues here that we must be sure to address. I'll give you my thoughts on each of them.
As before I recommend receiving in-person instruction as there may be serious technique problems at the root of this question.
Sufficient braking friction
A basic requirement is that your belayer is able to easily produce enough friction to arrest any fall you might take, be it one foot or fifty. (I sure hope I never experience fifty on either end of the rope but such things have both happen and are survived by others.) It is concerning for you to say: "she ... had a really hard time holding my weight, in fact she could barely hold it at all." In the mock-up you described it should be easy to merely hold your weight with one hand. Unless your fiancée is quite weak this must be due to the combination or rope, belay device, and braking position. A thin and slick rope will slide through a belay device with much less friction than a thick and used (fuzzy) rope, but the kind of rope you should be using for top-rope climbing (10mm or thicker) usually has enough friction in even the base-model ATC, so if you are using a 10mm or thicker rope first make sure that the braking position is correct.
Friction increases as the bend angle in the rope increases and bend radius decreases. The greatest braking force will be possible when pulling your hand(s) tight to your pelvis below the device, to the front. While gripping the rope tightly is important, especially at the moment of catching a fall and for hands that have not yet become strong from climbing, the primary focus should be on bending the rope across the lip of the belay device rather than gripping it more tightly.
Hands should be thumbs-up, that is thumb pointing toward the device, not away from it; NOT like this:
Be sure that your fiancee does not slide her hand all the way up against the belay device. The web of the hand can get "sucked into" the belay device; for a belayer of equal size to the climber this can result in a nasty pinch, but I heard that this happened locally to a new belayer paired with a much heavier climber (much greater size disparity than I believe you and your partner have) and it resulted in serious damage to the hand that required reconstructive surgery. Practice a consistent brake position at least several inches below the device indexed against the body rather than floating in space.
A device with more friction
If the correct braking position is being used and the belay device still does not provide sufficient friction you will need to change something. I recommend the Black Diamond ATC-XP (which I have experience with, though similar designs are available):
The V-shaped grooves greatly increase braking friction on skinny and slick ropes, and should let your fiancee easily hold your weight. Assisted-braking devices (like a GRIGRI) will also provide greater braking force but as I stated in a different post I am convinced that learning to belay starting with one of these devices is a mistake. They may however be a good idea for lead belaying where a size discrepancy exists. (See caveat below.)
If the problem is not lacking sufficient braking force but instead you are lifting your belayer off the ground you will need to attach weight to the belayer. My local gym provides sand bags for this purpose; they have an adjustable "leash" and people clip them to the belay loop (with an attached carabiner) with just enough slack so that it isn't pulling down while standing.
The amount of friction in the effective pulley at the top of the wall will affect the allowable weight discrepancy before this becomes necessary. From experience I know that the tipping point on a large-diameter anchor is right around a 3:2 ratio; if your belayer weighs two thirds or more of what you do it should not be needed.
Consideration for lead belaying
I have limited experience with lead climbing at this time. Nevertheless I can tell you that catching a lead fall is a completely different experience from catching a top-rope "fall." Even a moderate fall by a climber your own size will pluck you well off the deck. Beyond that I shall pass on making recommendations regarding size disparities in lead climbing as I'm simply not qualified. However I wish to firmly impress on any top-rope climber reading this that lead climbing is an entirely different game, even for the belayer. Get well-qualified instruction before stepping into this realm.