There are lots of ways to make smaller weights work a little harder for you: adding pulses, slowing down your reps (4-8 counts instead of the standard two) and changing up your grip/angles/plane of motion etc. But eventually, you’ll probably want something that’s going to really challenge you and keep your results coming.
With low weights (under 30lbs), our learning/adapting curve tends to be shorter. While the 15’s might seem a little too heavy for the rep work you’re used to, it’s likely that using them for 1-3 weeks (and cutting down your reps), will get you adjusted fairly quickly. If your current rep range is 12-20 (which is high, but common for many women), think about lowering it to 8-12 with the heavier weight. You can also do split weight sets: use the 15’s to start, then switch to the 10’s when you need to (this is an option I often advise for people transitioning weights with follow along DVD’s). Starting heavier, or swapping your load throughout, can help you adjust without slowing you down too much. You can continue doing this until you’re ready to do the whole thing with the new weights.
If you have access to a gym, or are buying in store, give it a try on the spot. If you can do 8 (even 7) but struggle at 12, it’s probably the right dumbbell weight for you. You’d be surprised by how quickly we adjust to a slightly heavier load: it won’t seem too difficult for long.
Another option (which I think is FAB for your situation by the way), is to invest in adjustable dumbbells. It’s one set, and you can adjust your load quickly and easily by adding or removing weight from each dumbbell. It’s a space saver (bonus), & it’s awesome for people who want to invest in something they can GROW with over time. Those who need lighter weights for some exercises, but can handle more for others (bicep curls vs. shoulder presses for example), might do better with one adjustable set than with several different pairs of weights.
There are plenty of fancy/expensive models out there, but for the weight range you’re looking for, there’s at least one option that’s only about $40 from Amazon. It’s one set, with adjustable plates, and it goes up to 40lbs (up to 20lbs for each dumbbell, or you can use just one and go slightly heavier). Check it out here: Cap Barbell 40-Pound Dumbbell Set
Hope this helps! I know the feeling! Generally, it’s good to have several different dumbbell options: one light, one medium and one heavy. Different exercises call for different loads, and on your ‘light’ workout days, you may want something lighter to play with (my 7-10lb weights are often a part of my cardio). Even if you end up with the 12’s, eventually they may become your new ‘light’ weight. The investment won’t be in vain! :)