How did Dr. Jekyll begin to lose control in The Strange Case of Dr. Jekyll and Mr. Hyde?

If we read the final chapter of this fascinating story, which gives us Dr. Jekyll's own account of all of his past and the relationship between himself and Mr. Hyde, we see that once he had began to take the transforming potion, it became incredibly hard to stop it. Even when Dr. Jekyll makes a conscious choice to choose "the elderly and discontented doctor" over the evil Mr. Hyde, and embarks on a determined course of action to note take the potion, it is clear that the temptation to embrace his darker, evil side is too powerful:

For two months, however, I was true to my determination; for two months, I led a life of such severity that I had never before attained to, and enjoyed the compensations of an approving conscience. But time began at last to obliterate the freshness of my alarm; the praises of conscience began to grow into a thing of course; I began to be tortured with throes and longings, as of Hyde struggling after freedom; and at last, in a hour of moral weakness, I once again compunded and swallowed the transforming draft.

Dr. Jekyll likens his cravings for the potion to an alcoholic thirsting after drink, and we see that having experienced the evil side of his personality, this part of his identity increasingly overpowers the good side of himself. Dr. Jekyll admits that he completely underestimated the "complete moral insensibility and insensate readiness to evil" of Mr. Hyde and thus he recognises that he is fighting a losing battle.

How did Dr. Jekyll begin to lose control in The Strange Case of Dr. Jekyll and Mr. Hyde?

Dr. Jekyll is experimenting with mind-altering drugs, and like millions of people who experiment with drugs he begins by feeling he can control them but later comes to realize that they are controlling him. This can happen to people who begin by smoking marijuana or using heroin, cocaine, or any other drug. It can also happen to people who start drinking liquor. And, tragically, it happens to millions of people who start smoking cigarettes. Dr. Jekyll is only an extreme case of drug dependency.

In he last chapter of the book, "Henry Jekyll's Full Statement of the Case," Dr. Jekyll explains how he began his experiments out of scientific curiosity and a desire to benefit mankind and how he gradually lost control of his identity as the evil side of his nature overpowered the good side.

I do not suppose that when a drunkard reasons with himself upon his vice, he is once out of five hundred times affected by the dangers that he runs through his brutish physical insensibility; neither had I, long as I had considered my position, made enough allowance for the complete moral insensibiltiy and insensate readiness to evil, which were the leading characters of Edward Hyde.

The Strange Case of Dr. Jekyll and Mr. Hyde became a world-famous classic because it is based on a fundamental truth about human nature. There are good and evil sides to all of us, and we need to exercise common sense and self-control in order to avoid the danger of having the evil side acquire ascendency. This often happens to young people who fall under the influence of bad companions.

Does Dr. Jekyll wish to control his darker side in The Strange Case of Dr. Jekyll and Mr. Hyde?

Dr. Jekyll acknowledges that all of us, even people who are basically morally good, have a darker or evil side. In his case, though, despite an appearance of respectability, he has a taste for depravity and various vices. Rather than trying to suppress the darker side of his personality, work on self-control, develop self-discipline, or become a better person through practicing charity, he develops drugs which enable him to split the good and bad parts of his personality into two separate people. He creates Mr. Hyde out of the evil side of his personality and then uses drugs to suppress Mr. Hyde so that this evil alter-ego can remain hidden, along with Jekyll's worst impulses. Unfortunately, it turns out that evil, when suppressed this way, becomes stronger, and it becomes increasingly difficult for Dr. Jekyll to prevent Hyde from emerging and even taking over.

Does Dr. Jekyll wish to control his darker side in The Strange Case of Dr. Jekyll and Mr. Hyde?

Absolutely!  Just as he created a potion through experimentation to release the evil side of his nature, Dr. Jekyll is attempting to create an antidote which would prevent this side from getting out of control and possibly to prevent Mr. Hyde's appearance at all.  As Mr. Hyde becomes more powerful and consumes Dr. Jekyll's body more and more often, Dr. Jekyll realizes the seriousness of the situation and knows that he must make a desperate attempt to correct the problem.

Does Dr. Jekyll wish to control his darker side in The Strange Case of Dr. Jekyll and Mr. Hyde?

Yes, I believe that Dr. Jekyll wanted to control his other side, Mr. Hyde, and he was able to do so for some time; however, as time went on, he didn't need to consume the formula to become Mr. Hyde.  He had lost the control that he wanted to have!  Mr. Hyde was overtaking Dr. Jekyll, and Dr. Jekyll realized that this was a horrible situation to be in.  He realized that he had to ride himself of Mr. Hyde or he would be forever doomed.  

Does Dr. Jekyll wish to control his darker side in The Strange Case of Dr. Jekyll and Mr. Hyde?

Although Dr. Jekyll is, at first, fascinated with the idea of man being a combination of both good and evil.  After he has created the potion that can release th evil side, Mr. Hyde, Dr. Jekyll becomes desperate to control the decadent, murderous side of his nature. After the murder, in Chapter 4 Dr. Jekyll swears to Mr. Utterson that he is done with Mr. Hyde. 

"No," said the other.  "I cannot say that I care what becomes
of Hyde; I am quite done with him.  I was thinking of my own
character, which this hateful business has rather exposed." (Stevenson)

In Chapter 6, the author tells us that Dr. Jekyll seems to be rid of Hyde, his good health and happiness return, he is once more a social person. 

"The death of Sir Danvers was, to his way of
thinking, more than paid for by the disappearance of Mr. Hyde.
Now that that evil influence had been withdrawn, a new life began
for Dr. Jekyll. " (Stevenson)