It seems like common sense that you would need to lift heavy in order to get stronger right?
Well when I look around a gym, most people just do not grasp the concept.
In order to get stronger, one must lift heavy (85-100% of 1 rep max). If you do not know what your 1 rep max is, use weight you can lift 1-5 reps.
Most people like to stick to the "comfortable" 8-12 rep range. While this may lead to an increase in muscle mass (if a surplus of calories is eaten), but it is extremely hard to make large strength gains at this rep range.
Increases in strength largely come from neural adaptations. Heavy loads 85-100% of your one rep max, cause the greatest increase in neural motor unit activation and therfore cause the greatest increase in strength.
If your goal is to increase strength, the focus of your program should be on heavy (85-100% of 1RM) compound exercises (think squats, deadlifts, presses) for 1-5 reps.
Lastly, always look to make progress as progression is key to continually getting stronger. Add weight to the bar, do the same weight for more reps, or increase range of motion during lift to stimulate progress.
So stop using the pink dumbells, move outside your comfort zone and lift something heavy if you really want to see an increase in your strength. Progress happens outside of your comfort zone.
She's probably stronger than you. Lift heavy.
Till next time,