What should I do about the intrusive obsessive thoughts that I suffer from and have started to dwell on, and will I be held accountable for them?

It is by Allah's grace and mercy that He overlooks whispers [waswas] as long as they are not believed in or acted upon. These are among the pardoned matters for which one will not be held accountable. The servant's dislike of these whispers is an indication of the soundness of their belief and the strength of their faith, and striving against them is an act of worship for which one is rewarded.