We understand that you're going through a challenging time in your relationship. Firstly, we want to remind you that God's design for relationships is holy and sacred, and He desires that we honor Him in all our ways. It's important to note that the Bible teaches us that marriage is a lifelong commitment between a man and a woman, and that intimate relationships outside of marriage are not pleasing to God.
You mentioned that you're living comfortably with your girlfriend, and this is not in alignment with God's will for intimacy outside of marriage as per the Bible. It's clear that financial blessings can sometimes lead to complacency, and it's important to remember that our ultimate trust and dependence should be on God alone. Let us look at the warning that Jesus gives in Luke 12:15 WEB: "He said to them, Watch yourselves, and be careful of all covetousness, for a man's life doesn't consist of the abundance of the things which he possesses."
Regarding the weight gain, it's important to remember that our bodies are temples of the Holy Spirit, and we should honor God with our bodies. However, it's crucial to approach this subject with sensitivity and love. Nagging or criticizing will not bring about positive change. Instead, encourage and support her in pursuing a healthier lifestyle. Remember it is only God that can change a heart and mind.
1 Corinthians 6:19-20 WEB: "Or don't you know that your body is a temple of the Holy Spirit which is in you, which you have from God? You are not your own, for you were bought with a price. Therefore glorify God in your body and in your spirit, which are God's."
However, the most important aspect is that your relationship is currently misaligned with God's design for intimacy and marriage. We encourage you to pursue a relationship that honors God and consider the commitment of true marriage as a lifelong commitment. We also encourage you to invite your girlfriend (future spouse) along this spiritual journey.
We must also address the fact that your prayer is asking for strength but does not reference the name of Jesus. We must access God by the name of Jesus. There is no other name by which we can be saved. Let us remind you of this is Acts 4:12 WEB, "There is salvation in none other, for neither is there any other name under heaven, that is given among men, by which we must be saved!"
Let us pray together:
Dear Heavenly Father,
We come before You, lifting up our brother in need of Your guidance and strength. Lord, we pray that You help him to grow in his love for You. We pray for Your conviction and guidance in his relationship to align it with Your will. Help him to love his future spouse as You have loved us, and to seek You first in all things. We pray for wholeness and holiness in this relationship and future marriage. Please help his future spouse put her hope and trust in You. Help her to honor You with her body.
We pray this all in the precious name of Jesus Christ, our Lord and Savior and the only mediator between God and man.
Amen.
Lastly, we encourage you to continue praying and studying the word of God and share it together. Let God be the center of your relationship and future marriage, and He will guide you both in His perfect will.
Prayer Focus: God, Thank You. Thank You for loving me. God, I ask You in Jesus’ name bless me with everything that I stand in need of and everything You want me to have. Bless me to know You in truth, fall in love with You with all my heart, mind, soul, body, and strength and never fall out of love with You. Bless me to have an ever growing closer stronger, more intimate relationship with You. Bless me with the love, desire, strength, and the spirit of obedience to always delight myself in You, seek first Your kingdom, and Your righteousness. Help and strengthen me God to always respect and obey You. Bless me to trust You with all my heart, acknowledge You in all my ways, and lean not to my own understanding. 
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