I actually have read conflicting studies on whether your metabolism slows down as you age or just plateaus between 20 and 60. There are just too many factors to take in to account during a 10 year gap that could be the reason for the change - eating habits, exercise, daily routine, stress, work, life in general. Anything and everything can effect the body in undiscernible ways.
Regardless, patience is your friend. One month is not enough time to judge the effectiveness. Most people don’t even start to see any results until at least a month. I suggest you just keep at, be patient, try and stay hydrate, and judge how your body feels and not what the scale says, at least for the near future. Remember, this is a lifestyle and not a fad diet.
That’s great that you’re back on track and motivated to get healthier. I suggest just giving it more time (3-9 months) and being patient. Slow and steady plus consistency will get you there. Also look for non scale victories, like fitting better in your clothes for example. My tip for help with drinking water would be these flavored packets for water by the brand True Lemon. I suggest going to their website and ordering directly. They have several flavors besides just lemon and some are sweetened, other unsweetened and they even have some for kids that are yummy.As far as your medication, I do not have personal experience and like you I’ve heard that they can slow progress/ contribute to weight gain but that’s okay. If they help you feel better that’s what’s really important, keep going you’redoing awesome.