Will Allah hold me accountable for the impurity of the semen that fell on the toilet seat in the college four years ago, knowing that it is difficult to remove now, and will He forgive my sin?

The cure for obsessive thoughts is to disregard them and pay them no mind. Semen is pure according to the majority of scholars, so there is no impurity that requires removal. Therefore, stop thinking about this matter, and repent to Allah with sincere repentance from the blameworthy habit. Do not pay any attention to obsessive thoughts of any kind, for yielding to them leads to great evil.