The most important thing I can tell you is that God doesn't want you to be this way -- and because of that, He has provided you with everything you need to gain a strong faith.
I've often asked myself why so many people are like you are: Their faith is like a roller coaster, constantly going up and down but never staying firm and settled. One reason, I've found, is because we confuse our faith with our feelings. Feelings come and go; sometimes we feel good, and sometimes we feel down. But if our faith is only as strong as our feelings, then it grows weak when we don't feel good.
Don't just trust your feelings; they will fade. Instead, trust what God has done for you in Jesus Christ, and trust the promises of His Word, the Bible. In other words, learn to trust God's truth -- for He cannot lie! Trust the truth that Christ died for you to forgive your sins, and trust the truth of His resurrection that assures you of eternal life. Trust also His promise that "I am with you always, to the very end of the age" (Matthew 28:20).
If you have never done so, ask Jesus Christ to come into your life today -- and He will. Then make the Bible a regular part of your life, as you hear it taught and preached in your church, and as you turn to it personally. The Bible says, "Faith comes from hearing the message, and the message is heard through the word of Christ" (Romans 10:17).