My problem is that I cannot complete anything I start to do, whether I actually begin to do it or I just intend to do it, and whether this action has to do with my acts of worship or my worldly life. Halfway through, I always stop what I had started doing. I hope that you can advise me.

Praise
be to Allah

The problem cannot be solved through advice and preaching;
rather what you are suffering from is a behavioural problem, which requires
a practical solution and remedy, and mere words of exhortation and verbal
advice will not be sufficient.

The first thing you should do to overcome this shortcoming is
clearly to persist until you complete the deed. Probably the first
successful experience will motivate you to keep going until you complete all
your deeds and do them perfectly. What will help you to achieve that is two
things:

1.    

Dividing the deed into small steps, so that you will feel more motivated to
complete each stage on its own, and keep going until you reach the last
stage and complete it. That is because human nature finds large, lengthy
tasks burdensome. So there is no alternative but to break the task down into
stages until you achieve it.

2.    

Choosing short and quick tasks and beginning with them, so that you will be
able to complete them. The Prophet (blessings and peace of Allah be upon
him) said: “O people, you should only do deeds that you are able for, for
Allah does not grow weary but you do. The most beloved of deeds to Allah is
that which is done persistently, even if it is little.” Narrated by al-Bukhaari
(5861) and Muslim (782). And he (blessings and peace of Allah be upon him)
said: “The most beloved of actions to Allah are those which are done
persistently, even if they are little.” Narrated by al-Bukhaari (5861) and
Muslim (783). 

Imam an-Nawawi (may Allah have mercy on him) said: In other
words, that which you will be able to do regularly without it causing you
any harm. This indicates that it is encouraged to be moderate in doing acts
of worship and to avoid taking on too much. The hadith is not speaking only
of prayer; rather it is general in meaning and applies to all righteous
deeds. End quote.

Sharh an-Nawawi ‘ala Muslim
(6/70-71).

We advise you to read some books and psychological studies,
which you can benefit from regarding this matter. Such books include: al-Futoor,
Asbaabuhu wa ‘Ilaajuhu by Shaykh Dr. Naasir al-‘Umar, and ‘Ajz
ath-Thiqaat by Shaykh Dr. Muhammad Moosa ash-Shareef.

And Allah knows best.