Depending on the situation, as to why you're feeling bitterness. The Lord only knows the best way to deal with it, but I'll pray for you, and I'll ask Him to show you or someone what to do, and then share it with you. Somethings are just way harder to not have bitterness over than others, and God understands everything. However, remember that in the Bible, in Romans 12:14 it says Bless them which persecute you:bless, and curse not.
Also in Matthew 5:44-46 says But I say unto you, Love your enemies, bless them that curse you, do good to them that hate you, and pray for them which despitefully use and persecute you: That ye may be the children of your Father which is in heaven: for he maketh his sun to rise on the evil and on the good, and sendeth rain on the just and on the unjust. For if ye love them which love you, what reward have ye? do not even the publicans the same?
So remember those verses, and verse 46 is a key one here, it tells us if we love those that love us, what do we get? We know they love us already, BUT if we love those that don't love us (our enemies or whatever) then we recieve a reward(blessing) for it. Why? Because it's so easy to love those who love us, however it's so hard to love those that are against us, or have hurt us, or something along those lines.
Not sure if it did or not, but hopefully this helped some anyways. Feel free to message me anytime about this or anything else you need prayer, help, or advise on. God bless you!