What weakness can make us strong? How can falling into sin strengthen us in the future? Falling into sin (weakness) is a wound to your spiritual self, a deep gash; and, after being wounded comes healing (reconciliation and penance) and tough scar tissue (wisdom from the experience). Scar tissue is stronger than the original, as the experience of falling into sin should make you stronger and wiser to avoid falling in the future.
Your “self”, your “flesh”, will always be seeking worldly comfort and happiness and, mostly, at little strenuous activity. Achieving the habit of saying “no” to your “self” is successful discipline. But, you cannot do it on your own. So, what do you do? The fortitudinous man draws strength from Jesus Christ.
One of the greatest means for fighting against temptations and assailments from the enemies of our souls is not in focusing on them, but on keeping our eyes solely focused on God and nothing else. A famous spiritual writer by the name of Brother Lawrence wrote a series of letters that were collected into a book called The Practice of the Presence of God. In it he does not go into all the details of devotions and penances, but constantly reminds us of the first thing in the spiritual life, which is seeking God above all other things. Amazingly, by focusing on Christ and not our own strength, we gain strength to focus on Christ. Here’s a good summarizing quote:
Take courage, offer Him your pains incessantly, pray to Him for strength to endure them. Above all, get a habit of entertaining yourself often with GOD, and forget Him the least you can. Adore Him in your infirmities, offer yourself to Him from time to time; and, in the height of your sufferings, beseech Him humbly and affectionately (as a child his father) to make you conformable to His holy will.
From The Rule of St. Benedict:
The second degree of humility is that a person love not his own will nor take pleasure in satisfying his desires, but model his actions on the saying of the Lord, “I have come not to do My own will, but the will of Him who sent Me” (John 6:38). It is written also, “Self-will has its punishment, but constraint wins a crown.”