How can I prove it

My wife and I have been apart for a year now because of her crazy thoughts.i loved her very much and always will. I was in a very dark place for a good few months and we weren’t even speaking.from having a normal family life one minute and then having taken away without rational reason the next nearly destroyed me but I finally started to get my head around it and then we started speaking again which was good. I know we won’t be together any more and finally excepted that about four months ago.she has since told me she believes I am the father of another child between the age of our children and I volunteered to have a DNA test to prove she is incorrect,she is not interested in the test because in her mind she is correct.the dark feelings have come flooding back and this time they are even worse.i try to put them out of my mind but they creep back and I can’t control them,help me,I don’t want to feel this way again,I can’t go on and won’t go on like this.i thought there was light at the end of the tunnel but the switch has been turned back off.