25 And when ye stand praying, forgive, if ye have ought against any: that your Father also which is in heaven may forgive you your trespasses.
26 But if ye do not forgive, neither will your Father which is in heaven forgive your trespasses. Mark 11:25-26
But should I forgive because it's a command of because my heart forgives
@t-flames, forgiving others is not about whether your heart lead you to forgiv or not! Infact, as human, you are to use your mind (heart) to decide things and not your heart deciding for you. Unless you are led by the spirit. Most times the mind can even tell you to do things that are not good. I'm dwelling more on this because naturally, the heart can tell you not to forgive atimes or even keep something in mind against someone.
However, you forgive because is a must (command) that you forgives whosoever offends you whether they apologize or not; because without forgiving them, there is no way God can forgive you your sin whenever you ask Him for forgiveness!
When you forgive people, if need be, you have to let the person in question know that you have forgiven him/her.
Therefore, You must forgive everyone that offends you or anyone that you have something against in your mind before God can hear your prayer of forgiveness.
We all know that God is a merciful God but you have to obey His commandments and do His will to receive His mercy. Remember that He said in His word that "He will have mercy on whosoever He choose to have mercy on". So you see, God's mercy is not for everyone but for whosoever He choose.
For example, God cannot have mercy for the devil, no matter how he praise God.
In a summary, unforgiveness is a hindrance to answered prayer!