You may be madly in love with your boyfriend, but this relationship is doomed. If you can’t find it in your heart and mind to love this little girl, who you describe as a “good kid,” you can’t expect to make a life with him. You can’t expect him to choose between the two of you. It’s unfair to make her the scapegoat for whatever your own reservations are about moving forward in the relationship.
You say you’ve already seen two therapists. Perhaps it’s time to try a third.
Sometimes therapy is like opening a pickle jar. Two people try it and can’t open it but the third succeeds. It’s not necessarily that the third is stronger or better at it. The first two kind of loosened it up so the the third could give it the final tug.
Do take your letter and this response with you. And sign a release so the therapist can talk to the other two. That will jump start the process so you don’t have to completely start over.
I wish you well.