Will Allah reject my supplication because of my sins and my thoughts of forbidden things, which are delaying my marriage, and how can I completely rid myself of sins and transgressions?

Merely thinking about a sin does not hold a person accountable for it unless it is followed by an action. And it is from Allah's mercy that He overlooks what we merely think about, as long as we do not act upon it or speak of it.

One should be diligent in guarding their thoughts for the soundness of the heart and the purity of the chest, and avoiding such thoughts is better.

Sins should not prevent one from supplicating. And let the first thing supplicated for be well-being from sins and repentance from them.

Have a good opinion of Allah and be certain of His response. Seek out times when supplications are answered, strive in obedience, and repent to Allah whenever you sin.

Among the things that aid in steadfastness are: accompanying righteous people, adhering to the remembrance of Allah, striving against one's self in acts of obedience, and avoiding disobedience.