So pleased that your are seeking proper support, also ask your GP about Counselling. I was offered this by my GP when my son died, but refused it - I only wanted to talk to people who had known him, and - fortunately, back then I did have lots of friends, etc. who were willing to let me ramble on about him.
It does amaze me, quite often, how many people have had adult children who have died. Even chatting to strangers, on trains, etc. I talk about my son, and so often these people will tell me of a similar tragedy in their lives.
Each person does need to deal with this in their own way - but it is important that whereas one life is ended, it does not mean further ones should (effectively) be so. One of my twins did need to have therapy and counselling for nearly a year after his death.
Nearly eighteen years later, we still miss him, and think about him, but the sharpness of that greif is subsiding. When I moved, the very first photo I put up in my new flat was his!!!
Anniebach, when your daughter next visits, or - perhaps -with one of the people from your grief group, get them to go with you over that bridge. Accept it as part of her life - and do not let it rule yours.
Think you need to stay in your present home - at least for a couple more years to go through the worst of your grief - took me just over two years before I actually began to feel I was coming out of that black cloud.
Would be good if you could have the occasional holiday in Lincolnshire with your daughter and SiL. Forgive him for just dropping your daughter off that dreadful day - perhaps he felt he would be intruding on close family if he came in that dreadful time.
Thinking of you
My four year old grandson "doesn't want me"