How i can know that G-d has forgiven my sins?

Shalom, Thank you for your question. The concept of forgiveness is a major one in Jewish thought. The Torah expresses this a great mitzvah – and all the prophets spoke passionately about it. So, you are certainly asking about an important topic. When it comes to forgiveness, Judaism teaches that there is a process that the sinner needs to undertake in order to attain Devine atonement. That process differs from sin to sin – but in general, with sins that harmed another person, firstly one needs to make amends to that person. This may involve a monetary payment (as in the case of damages), and nearly always involves asking the person for their forgiveness. Then, and only then, one must turn to G-d and ask for His forgiveness. This is done by a private verbal confession. Then one needs to undertake to not repeat the sin. If the sin was between man and G-d (such as breaking Shabbat or the Kosher laws etc), then after feeling regret at their actions, one turns to G-d in the same process. Through this the process of forgiveness is performed. The total atonement for the sin also depends on what the exact sin was. Different sins have different processed of atonement – but in general this process is performed by G-d, and not the person. For some sins one needs to undergo the fasting on Yom Kippur. For other more serious sins G-d also sends suffering (in this world or the next) as a means to achieve atonement. When we will merit to have a Temple again, certain sins require atonement sacrifices. In general – one should do their best to undergo the process of forgiveness (Tshuvah) and with G-d’s help, they will merit to achieve a total atonement as well. Blessings.