Shalom. Im writing this urgent question in the time of great sadness and depression. When I was doing one avirah, I though to myself "If I will do this, Im losing my part in Olam HaBa". Not in general punishment for sin manner, but neder-like. It was only a thought, and no Shem Hashem involved in it. As you can understand, I ended up doing it. The question is, whether this is an actual oath, and if yes, am I losing only part on Olam HaBa that I have already earned, and whether hatarat nederim will reverse this.

Shalom, Thank you for your question. You should not fear – and let nothing stand in the way of Tshuva. Your thoughts did not add any type of vow, or any form of restrictions on your ability to return to Hashem. The opposite is true – they are an indication of the true thoughts of your soul. That it does not want to be separated from Hashem through sin at all. This should give you strength to do tshuva and withstand temptation to sin in the future. The fact that your inner voice calls out, even in the moment of sin itself, is a true sign that you are a holy soul that wants (and will, please G-d, achieve) a good sin free life. May you blessed with every success. Blessings.