Belief is a choice. That's what free will is all about! We all have a choice to believe what the Bible says, believe Jesus died for our sins, and believe he was raised from the dead. If God forced us to believe then we wouldn't have free will. What God does is give us faith to believe once we've made the choice to follow him.
Personally I believe if you are sincere when you say the sinner's prayer, and accept Jesus as your savior, accept the gift of forgiveness, then you are saved. What you do with this gift is what is important. We follow Jesus for what he has done for us and strive to be like Jesus who is our example. All God wants is your thanks, recognition of Jesus work, and an honest desire to follow him. We don't follow God out of fear but because we love him and appreciate what he has done to deliver us from damnation.
If you've confessed Jesus as your savior then you are saved! Jesus did everything that needed to be done to get you to Heaven. Many don't accept Jesus because they think it is too simple. They think- that's it?! Don't I have to read the Bible every day? Don't I have to do this or that? The answer is no.
Does this mean we can live like those who aren't saved? Once again the answer is no. We have a higher calling not to live like the world does. We prove our faith and what we believe by our lives. When we share Christ, help the poor, love our enemies, read the Bible, and try to avoid sin it shows we are Christians. It isn't the thing that gets us to Heaven but it's evidence we believe what we believe. I help that stranger because Jesus lives inside me and is what he wants me to do.
Your faith will grow the longer you stick with your faith. I'm 42 now and have been a 'true' Christian for around 20 years and it's still a learning process. When you first start it's easy to be swayed by the world and nonbelievers because you're ignorant of God and what the Bible says. Through time your faith grows and all the things people say don't matter.
There's nothing that could ever be said or done that would take away my faith in Jesus now. Which goes back to your original question is it a decision. It is even a decision to this day but I know so much now about God and Christ it's an informed decision. Every Christian could walk away tomorrow and worship the devil but we don't because we know it leads to destruction.
Amplified Bible (AMP)
23 So that all men may give honor (reverence, homage) to the Son just as they give honor to the Father. [In fact] whoever does not honor the Son does not honor the Father, Who has sent Him.
24 I assure you, most solemnly I tell you, the person whose ears are open to My words [who listens to My message] and believes and trusts in and clings to and relies on Him Who sent Me has (possesses now) eternal life. And he does not come into judgment [does not incur sentence of judgment, will not come under condemnation], but he has already passed over out of death into life.
25 Believe Me when I assure you, most solemnly I tell you, the time is coming and is here now when the dead shall hear the voice of the Son of God and those who hear it shall live.