Just a few possibilities
Your body does change as you get older and at 20yo it is possible that your body is still maturing through the tail end of puberty if you were a late bloomer.
Are you really seeing your body for what it is? Are you perceiving more flubber than is actually present?
If you are feeling healthy and in a healthy weight range, maybe this is more about your expectations about what your body should look like than expectations about health?
Have you done a body composition test to see what percentage of your body is fat as opposed to muscle to give you another data point to base decisions on?
Yes weight loss can take a long time, particularly when you have a relatively small amount to lose and you may have messed up your metabolism with yo-yo diets (that is, your metabolism may have called your bluff on the healthy eating and be waiting for you to change your mind again). You're also getting older (shock horror) which may well have shifted the goal posts.
What else has changed since it was "easy"? Changed jobs, house, routine, relationships.... so many factors which could have changed. I recall one person on here saying they realised that a small change in house or pets or something meant that a whole chunk of incidental exercise went missing.
35kg lost. (November 2005 - October 2006)
15kg gained again (as at October 2010).
Back to the drawing board - Let's do this thing!
"You can't change the winds, but you can change the sails"
"Reach out and take control of what lands in your lap"