How is it possible to change habits cultivated over lifetimes

The Pali Buddhist scriptures teach that present happiness & suffering of a person are not caused by actions performed in the past. Refer to AN 3.61, which states:

When one falls back on what was done in the past as being essential, monks, there is no desire, no effort, 'This should be done. This shouldn't be done.' When one can't pin down as a truth or reality what should & shouldn't be done, one dwells bewildered & unprotected.

Each experience can trigger your habitual response which might be followed by and intention to act. What triggers your habitual or reflexive response is and external stimuli coming in contact with your sense faculties.

So what you have to see is if your intention is good or bad. If it is good follow through else try to practice some restraint. When trying to restrain yourself you find then your habitual response overpowers you. But with practice you will be able to retrain your self when in intentions are not good.

Generally when your intentions are bad you have either unpleasant or neutral feeling / sensation in your body. If they are good it is pleasant or neutral. You do not get pleasant sensation if your motivation is bad and unpleasant sensations if your motivations is good. Neutral being a gray area. To get a better understanding see the 121 mind states and 52 mental contents. Refer to Comprehensive Manual of Abhidhamma by Bhikkhu Bodhi and Kītagiri Sutta section on feelings.

Paying conscious attention to whatever you are doing now fully, whatever the activity may be. Life is happening right now, right this moment as you read.

Any actions have consequences, even thoughts. Unconscious activities have unknowable/unforeseeable consequences. Habits are like unconscious activities. If you are doing a good thing why don't you do it with full attention? Why do you need "good habits"? Why do you need habits? Habit seems like you are using some body memory to just do whatever action and it will have whatever consequence.

Put yourself in control over this moment. The next moment does not exist except in your mind. Quell it. Your past lives don't exist except in your mind.

Don't create past lives in your mind. It does not know anyway. You have no idea about them. Habits are forming when you don't pay the right attention to whatever you are doing. For example, if I go to draw water from the well everyday, outside people may see it as a habit, but I am doing it for a particular purpose, we need water for some other activities. In the same way, look at your actions as doing something we need for the circumstances. Not as a habit.

Focus on whatever action you are taking now. Focus on life right now. Not past lives.

If a yard has lots of weeds, just by getting rid of the weeds alone won't be effective. One will also need to overseed their yard with strong grass so that they will overwhelm the weeds and grow into a healthy lush green lawn. Similarly, not only one needs to abandon bad habits, one'd also need to cultivate good habits so that they will "overwhelm" the bad ones. In Buddhism, it's called SammaPpadhana/The Four Right Exertions

It is not at all a simple task to reverse the habits cultivated over umpteenth lifetimes. It is next to impossible without gaining a good knowledge of the Dhamma and putting it to practice diligently. Then, and only then you are going to have any success in thwarting the mind’s ingrained habits. The personality view is the hardest of them all. If you could begin altering your habits, one day in the years to come you will be very thankful for getting into a path that made the impossible possible. Now it will feel like drug withdrawal of an addict when quitting the bad habits.

We can alter our habits because they too are impermanent. Consciousness too is impermanent and constantly changing. Thus, all these aggregates are constantly changing and impermanent. They are processes, not things. They are dynamic, not static. This knowledge will help us to see personal experiences in terms of processes, in terms of impersonal functions rather than in terms of a self and what affects a self.

You’re learning to use the tools that Buddhism offers to sort out your own mind. First the tools help you look at the habits, the things the mind is doing. Then goodness will come, not from any innate goodness in the mind, but from a keen sense of the dangers of the unskillful habits already there, and the benefits of the skillful habits that one can develop in their place. Once we have this understanding, we can overcome ingrained habits. This is why the Buddha said that heedfulness is the root of all skillfulness.

In psychotherapy, to root out ingrained tendencies in the mind one may trace things back to what you did as a child. The teachings of the Buddha instead gets you to focus on looking at your habits right now, as they keep coming back again and again and again. The Dhamma will inform you as to what is virtue, and what is not; what is blameworthy, and what is not; what should be made a habit of, and what should not. This will get you to civilize your eating habits by learning to wean yourself from your passion for the junk food of sights, sounds, smells, etc., and look instead for good food within.

Unlearning some old habits is not easy. After a while complacency sets in. Complacency is a form of sloth/torpor. Complacency can be said as lethargy in the body (sloth) and drowsiness in the mind (torpor). It is when the mind is not sleeping, but not awake either. So be aware of this twilight zone, where nothing of use is achieved. Buddha advises us to be aware of this enemy in life that would stop you from succeeding in life. So we have to make a habit of getting up early every day, being energetic. Energy arises when one has a clear-cut direction. One knows exactly where one is going and keeps at it. This is one to keep Mara’s armies at bay.

Also check Cetana Sutta 3 - S 12.40 to find out how we form our personal habits.

Easy. Well, easy in theory :) -- you need to get fed up! You need to get really sick and tired of your conditions, so sick that you cannot live like that anymore. You are so fed up that you are either ready to die - or to change your habits. So that desire to get out leads to iron intent to change.