I can only speak for me and say I can't quiite remember how I felt. We attempted to go through adoption six years ago, we have a birth dd who was 5 and we got as far as the open evening. I was not ready so we waited for another round of ivf with donor eggs (we had had one round with donor eggs, one attempt with my eggs and many attempts at iui).
Fast forward just over two years and we started the progress when dd was seven and adopted ds (3 at the time) two years later. In that two year space we had a fresh and frozen cycle donor ivf, both of which failed.
If this feeling has just suddenly popped back into your head then it may be due to stress or strain and a reaction to the enormity of what you are doing.
Are you normally the sort of person who sabotages your own plans, getting so far before stopping? Are you normally level headed and know what you want?
I can honestly say now, two and a half years into adoption, I am not sorry our treatment failed and we ended up adopting. But our situation is different to your situation and I know for some people who have not had a birth child it can be very difficult to accept. But for some accepting and moving on is absolutely the right thing to do.
If the feelings persist I would seek some specialist counselling. Because being found out doing something is more serious (in this situation) than actually admiring to doing it, IMHO I would tell the social worker you are thinking of having some counselling. As the counselling situation is private and confidential you should be able to do this without revealing too much, but you may need to put the adoption on hold to do this. So you must discuss with your partner (unless you are adopting alone) about all this before speaking to social services.
In your shoes, if I really could not get over that grief alone, I would probably speak to social services about dealing with the grief of not having a birth child and would use counselling to explore if this is right for me or not.
I don't know if you have had fertility treatment or not, and how much. We had a lot of expensive treatment and it all failed after dd (I was 39 and iui to have dd). So I am purely speaking from my perspective.