Solicitor has got it horribly wrong!

I jointly own 2 properties with my ex partner. We were not married and have 2 children she has a daughter from a previous marriage. I have 2 flats one which is rented and the other was rented but I now live in it, both are mine and she has not contributed to them at all.
I am still paying 1/2 of the mortgage of the property she is still living (£350 per month).
The house that she is living in is sold subject to contract and the other is rented.
I used a solicitor to severe the tenencies of both properties so we are 'tenants in common' and a will has been written so my 2 children benefit from my estate.
He wrote a letter to her indicating that on the sale of the property we were living in I would want my 1/2 share and other monies that were owed to me and the other property will be given to her.
Since this letter was sent to her I have realised that he did not mention that I could have equity in the other house, which I have now discovered is now worth £160,000 of which approximately £100,000 owed on mortgage. This would therefore owe me £30,000. He also got the money calculations wrong therefore leaving approximately £4,000 off my share!
I did not see a copy of the original letter and had to request this at a later date.
I wrote the proper terms down and asked him to write to her with the correct proposals. He did indicate that she would most probably stand by the 1st letter, which she has.
I have since aksed another solicitor to represent me and I have written to my other solicitor indicating my concerns and that I wish for him to no longer represent me.
Where do I now stand? I am at my whits end! The £30,000 I am potentially losing is my childrens inheritance and not my ex partners daughters!
Have I got any chance of this being resolved fairly?

Thankyou for your help Fiona.

I just need some piece of mind as he did not even advise me that there maybe equity in the other property or to get it valued!

Are there any other people who wish to comment also?