The simplest things aren't by any means the easiest things to get done. Just because they are termed simple it does not follow that they follow the dictionary definition of 'simple'. It is a common trap to tag any number of elements of daily routine, diet, or exercise change as merely simple. Changing a diet habit for example, sounds elegantly simple but is in fact often a feat of concentration, reinforcement, knowledge, and will. Changing your routine to fit in a few simple exercises, again sounds simple but requires saying "no" to other pulls on your time, reinforcement and enjoyment of the new routine.
It very often is not a self determined impression of scale, it is all to frequently classed that 'simple' thing because someone else said so (be it in a book, magazine, over coffee, on television). Very often these sources of reference don't describe the behind the scenes work, effort, repeated attempts, changes in approach, the do overs, the pit falls, and so on. Before interpreting something as simply 'simple' it has to be understood, understand that that early morning run will likely mean you'll need to go to bed earlier, understand that the change in diet to cut the caffeine has a number of pharmacological effects on you, understand that if you are making a single 'simple' change it sits within your bigger picture (and so will have an impact).
So what is my point? It is basically this, if you badge things as 'simple' but they just don't seem to work out - don't keep referring to them as 'simple'. Call them for what they are - tricky, difficult, awkward, challenging. Then having given your self the credit for identifying what a change means and that it will need hard work. Know that when you have taken it on and won it feels like a real victory and not just a 'simple' thing ticked off that no doubt long list of 'simple' things that should be done.
Give yourself the understanding of any challenge and that challenge will become something you can achieve.