Hi Rachel,
First, regarding knowing if you are a christian.. the enemy likes to attack our identity in Christ...
However Our Lord's Word tells us His promises in a few different areas.. His Word is the Truth, He reigns over all! I pray you cling to His Word and promises and fully know who you are Him, and He uses you for His Glory, in Jesus Name. Amen.
Romans 10:9
That if you confess with your mouth, "Jesus is Lord," and believe in your heart that God raised Him from the dead, you will be saved.
Romans 10:10
For it is with your heart that you believe and are justified, and it is with your mouth that you confess and are saved.
1 Corinthians 12:3
Therefore I tell you that no one who is speaking by the Spirit of God says, "Jesus be cursed," and no one can say, "Jesus is Lord," except by the Holy Spirit.
1 John 4:2
This is how you can recognize the Spirit of God: Every spirit that acknowledges that Jesus Christ has come in the flesh is from God, 3 but every spirit that does not acknowledge Jesus is not from God. This is the spirit of the antichrist, which you have heard is coming and even now is already in the world.
Regarding your struggle with sin.. Romans is an excellent book to read and re-read to help.. again, it is in His Word, the Sword of the Spirit, our Lord guides and directs us.. here are also some other verses and guidance in His Word
1 John 1
8 If we claim to be without sin, we deceive ourselves and the truth is not in us.
9 If we confess our sins, he is faithful and just and will forgive us our sins and purify us from all unrighteousness.
10 If we claim we have not sinned, we make him out to be a liar and his word has no place in our lives.
James 5
15 And the prayer offered in faith will make the sick person well; the Lord will raise him up. If he has sinned, he will be forgiven. 16 Therefore confess your sins to each other and pray for each other so that you may be healed. The prayer of a righteous man is powerful and effective.
17 Elijah was a man just like us. He prayed earnestly that it would not rain, and it did not rain on the land for three and a half years.
God bless you!