I've had anxiety / depression problems for a long time, and therefore it was no surprise when I got PPD following my daughter's birth.
I started back on my antidepressant 3 weeks post partum, and back on one of my anti anxiety meds (clonazepam which is in the same family of drugs as ativan) at about 3 months post partum. I was worried about the clonazepam making my daughter sleepy, but I never observed any impact on her. She has now just turned two and is an active and talkative kid (who still breastfeeds), I don't think the meds have had any effect on her.
Ativan is a medication with a shorter half life. If you are worried about the impact it has you can try to time when you are taking it so its peak concentration in your bloodstream (2 hours after you take it for ativan) is not a time you are likely to be feeding. I know when I first started back on my anti-depressant I was very concerned about passing the drug on to my baby and managing my doses so that the peak levels were at times that I was unlikely to be feeding (as best as one can when feeding on demand) helped assuage my anxiety somewhat. That having been said, the amount of drug passed through breastmilk is not high, and all that fiddling was more for my peace of mind than anything else.
I have dealt with intrusive IT both before and after the birth of my child. I have found that my best healthy tactic for dealing with them is to forcefully say out loud "STOP IT" when they start. That would give me a sharp interrupt and then I would continue to say "stop it" either out loud or in my head as required.
And the thoughts do go away. Things do get better. My PPD thoughts are gone completely. I had IT thoughts for about 10 years with my untreated anxiety/depression and those do reoccur depending on how well I am managing my illness, but I go months without them. So I would say that no matter how long you've been having these thoughts there is every reason to believe that you will be rid of them eventually.
Take care of yourself