If our choices are poor, it is because we are defining it as somehow less painful than another choice. Our fears and beliefs are either so painful to look at that we avoid them and the ideal choice by extension, and or it makes the ideal choice seem more painful than another choice.
Motivation can instantly be changed by looking to see what we perceive and believe to be the most pleasurable. Since these are determined by our own definitions, we can change them. Then our motivation is to choose what is closest to pleasure on that basis.
There are many potential reasons for why those choices are closer to pleasure than the choice to exercise, unique to the individual. However, I can think of several possiblities. We may believe ourselves to have limited resources of energy, and be afraid that working out will not allow us the energy needed for other activities. We may believe ourselves to have insufficient time in a day to do all the tasks we wish to do, and so may be afraid of not getting work or other tasks associated with a higher degree of pain done should we not do them. We may be feeling very anxious and choose the more immediate perceived pleasure of a donut in an attempt to resolve the pain of anxiety.
The first step is knowing what our believes and definitions are. The second is to see if those are what we really want, and whether those are really motivating us towards the choices we truly wish to make.
Fundamentally, beliefs must be examined and changed if they are not in harmony with who we wish to be. Sometimes this may require therapy or other forms of assistance, but in many cases you may find that you can work on them yourself too. Feelings must be experienced in order to gain the information the feeling is communicating to us.
Beliefs can be changed in a variety of ways. Ideally, they are changed by seeing that they are not in alignment with our selves and declaring something else to be true internally. Some beliefs can be challenged and modified through that process. For example, if we believe we don't have enough energy to work out, we can look at our past experience and see if exercise depletes us and in what contexts. Perhaps with that information we can choose a form of exercise that will be more likely to leave us refreshed rather than drained.
However, the most important thing to remember with beliefs is this: we will hold onto something as long as we believe it serves us. If they are leading us into choices we do not like, then they are not serving us and new beliefs are needed.
When it comes to feelings as information, here's one example. Perhaps we are feeling anxious, and this makes us realize we have chosen too many tasks to do today and that we are unclear on how we are going to do any of them. Thus it is really a call to clarify for ourselves what we are going to do and how we intend to do it. And if is too much, then seeing the plan of how we intend to do the tasks may allow the necessary reality check to make the adjustments needed.
Let's look at what might be a tougher case for some. Perhaps we feel unmotivated because we are feeling low or down. This may feel like a heaviness and perhaps even a pointlessness. So our choices may often be in a direction away from "accomplishments" and "productivity". I put those in quotes to remind you that these are personally and culturally defined concepts. Its up to each of us to determine what we consider a worthwhile use of our time, which may include doing things we don't completely like in order to fulfill a greater objective such as caring for one's family.
But with this down feeling state, we are looking at a whole host of limiting beliefs and fears. The case is often negative self talk along with feelings that are tough to face. Understand that I am not lightly saying just fix yourself somehow. But when it comes to motivation, it is important to understand that the beliefs and fears are sending us into choices that appear to be less painful. Even if we intellectually know another choice to be more pleasurable, there may be so much fear and doubt around our own capacities that it still appears as the more painful choice due to the pain of negative beliefs and fears.
So rather than bemoaning low motivation, try considering it a call to examine what beliefs and feelings are present within that are pushing us into choices we wish we were not making.
After all, motivation is always high. We are always motivated to choose what is closest to pleasure and furthest to pain.