Feelings are what tell us about what we want to do or not want to do, but it is volition that does the actual "doing". Action is more directly connected to feelings in non-human animals, although some form of "free will" is observable in the behaviours of the great apes.
Volition can decide what to do, but a cost is always exacted against if for the decisions that it makes. The greater the feelings against a decision, the greater the cost.
The concept of "energy" is a metaphor – it does not refer to energy in the physical sense. Our mind/brains do use up energy, but this is different to what I mean when I talk about "mind energy".
A better metaphor may actually be "money", but the expression "mind money" sounds a bit strange.
Although mind energy is not physical energy, it almost certainly has a physical representation (i.e. it's not magical energy). Neither I nor anyone else knows exactly what that representation is, although the neurotransmitters serotonin and dopamine are likely to be involved.
The "reward" that volition receives comes from unexpected satisfactions.
The "punishment" that volition receives comes from unexpected dissatisfactions.
The sudden death of a loved one is very painful, and will caused a depressed mood for an extended period of time. Unexpected good luck, like winning the lottery, can make you very happy. However, in both cases it is observed that eventually you get "used" to your changed circumstances – the survivor of a death in the family is not forever unhappy, nor is a lottery winner forever happy.
Tolle Eckhart is one of the more famous exponents of "living in the moment". This expression corresponds to the concept of volition acting with minimum resistance to current feelings. The advice to live in the moment makes sense when mind energy is at a low ebb. But there must be a reason why volition has the ability to act against feelings to a greater or lesser degree, which implies that acting as much as possible with feelings cannot always be the "best" thing to do.
Volition is judged in the context of current feelings. That is, the "cost" of exercising volition is a function of how much it is acting against or with current feelings. We may be tempted to align current volition with expected future feelings, but such a strategy is doomed to failure, because it performs the wrong calculation with respect to the cost of volition. This is not to say that we should not act with regard to the future, but only to the degree that we have feelings in the present about the future consequences of our actions. We cannot ignore the future, but at the same time we cannot force ourselves to respond to future circumstances as strongly as we respond to present circumstances.
If a goal requires persistent expenditure of mind energy, then by the time it is achieved it is already an expected result. So no mind energy is gained from its achievement, even though mind energy was persistently spent in the effort to achieve the goal. A goal that requires this type of persistent effort will only ever be achieved when your feelings about it are strong enough that they tell you in the present moment to act towards that future goal.
The solution to this paradox is to expend mind energy on doing things that are likely to have unexpected benefits. This might be meeting new people, or going new places, or trying new ideas. A similar rule applies to the downside, in that mind energy can be spent on preventing unexpected disasters.