The true reason we must forgive others is that our salvation is based on faith. Jesus died for you and all you have to do to take advantage of that is believe that His sacrifice was enough to pay for your sins. There’s a catch, though: He died for everyone else, too.
In other words, if His sacrifice is for everyone, then you have to choose whether to believe it’s powerful enough to forgive everyone or to forgive no one. There’s no middle ground third option here. Because the root of every sin is choosing yourself over God, the actual mode in which you choose to sin, be it cussing someone out or killing someone, is irrelevant as far as the judgment from God you earn for yourself. One sin, one choice of you instead of Him, is a choice of death everlasting instead of life. And we have all made that choice, most of us many, many times.
This makes your faith in God to forgive others and yourself the same. If you choose not to forgive others, you put yourself in the Judgment Throne of God (which is not exactly your rightful place) and essentially say that Jesus’ blood is insufficient to pay for their sins. If you believe that, then you concurrently believe it’s not enough for your sins, either. Differentiating between your sins and theirs doesn’t work because it’s not your judgment of the importance of the sins that counts; it’s God’s right to judge.
God wants us to have faith in Him and in His Son. We don’t have that faith if we have grudges. And if we don’t have that faith, how can we expect to be forgiven? Let go of your grudges and resentment against others and yourself. Once you truly believe that all sins are equal in God’s eyes and that you have the chance to be forgiven for many times more than what you’re forgiving, forgiving others will come naturally, especially as your faith in these things grows.