God is almighty. Therefore God can do anything. To believe God could not forgive could be an even greater sin than anything a human could do, as that would mean you doubt that God is almighty. Therefore, believe that God can forgive anything!
Jesus spent his life with sinners, thieves, adulterers, and the like. Jesus taught us that the question is not what one has done; the question is, have you repented? If you full-heartedly repent it, then God will forgive.
Do you know the TV-series My name is Earl? In that series, a petty crook discovers “karma,” the spiritual concept that if you do good, good will be done to you. Earl did only bad things, and he decided to make up for every single one of them. He writes a list of all the bad things that he has done (there are many quite ridiculous ones). He approaches those who were involved or affected by his bad deeds and asks them how he can make up for it so that he can cross them off his list. Everyone wants something different, and he tries in multiple ways to give something back to the people. Despite the bad things that he has done, they really appreciate his change in behavior. It takes guts to even approach those people, and some stories are pretty remarkable.
Almost all of us did some things in life that were not at all good and that we are not proud of. For myself, I am inspired by Earl and thus created a list, and I am working on making up for it. Can I just tell you: that feeling when making up for something bad is unbelievably good! Thick stones fall off my heart and my conscience lightens up immensely every time I resolve something.
For example, while I was studying, I was visiting a bar with some colleagues. We were sitting outside, drinking and having a good time. On the chairs outside, they had nice blankets to warm their guests. We agreed to take one of those nice blankets with us, secretly. As the blanket was at my house, every time I looked at the blanket, I had a bad conscience. Such a blanket costs something, and if everyone just took one, the bar would not offer them anymore, which would be a pity. It is not my blanket, and it feels wrong to have it.
I searched for the bar on the Internet and wrote to them. I explained the situation, said I was sorry, and asked them how I could make up for it (e.g., send the blanket back, pay for it, etc.). Sending that message felt very good. The answer came promptly. They thanked me for the honesty but said that to save the environment, I should not send the blanket back but donate it to a charity close by, and I should invite my friends for a beer and tell them about their great bar. I felt extremely relieved. I donated the blanket and told my friends about the great bar! And if you ever visit Malmö in Sweden, go to best bar in town—the Moosehead Bar (https://www.facebook.com/MooseheadBarLillaTorg/). However, kindly leave the nice red blankets where they are!
You can make up for anything if you truly repent. Try it. Take all your guts and just do it. It will feel wonderful and lighten up your soul!
In case somebody that you did something unjust to that has died already, ask their descendants how you can make it up for the sorrow of their loved one and pray to God for forgiveness. As long as you are living on this planet, it is not too late to turn around and change for the better!