Sterilization is mutilation of the body, the temple of the Holy Spirit, and hence a matter of mortal sin. It also intentionally closes one off to life, which is indicative of someone closing themselves off from God. Sterilization, for contraceptive purposes, can never be done under any circumstances.
You may be weak in the flesh (as most people are), but this is No excuse to give up and resign yourself to a life of mortal sin. Such a resignation:
- insults God
- what He has done for each of us on the Cross, and
- the grace He offers us in the sacraments.
I firmly resolve . . . is what the Act of Contrition prayer says. With the help of His grace that resolution is what it means to be a Christian. Mother Teresa once said,
Christ doesn't call me to be successful, He calls me to be faithful.
Part of that faithfulness is believing His promise that he will provide the grace for you to eventually overcome your sins and to be made whole, if you persevere with Him.
Masturbation, contraception, and sterilization are all grave evils and matters of mortal sin.
How guilty one is depends on the knowledge and freedom one has at the time; but to give up and resign the fight is to reject:
- His Son
- His Church, and
- His grace.
Morally speaking, you cannot do one in order to avoid the others. That is objectively evil. My advice is to not go through with the sterilization and to get on your knees and beg God to enlighten you as to how best you can:
- cooperate with His grace, and
- walk with Him through these serious struggles.
God loves our resolution, honest effort, and perseverance to try to please Him and overcome our brokenness due to sin.