Welcome to beyond blue.
Firstly I want you to know that I am listening. Secondly, that you are already seeing a therapist should be seen as good news as it sounds like you want to the address this problem which is a good start.
It sounds like you are overwhelmed by the rollercoaster ride of emotions that you go through each day with your boyfriend. Trying to stop the thoughts you are having might take time. It would be nice to be able to just turn them off. But we cannot.
Does your boyfriend know you are seeing a therapist? Have you been able to explain to your boyfriend about how you are feeling?
I am not sure what your therapist has suggested... and my mind generally races about different things. It feels like there are 10000 things going on, when in reality there are only about 3-5 things, but those things you see from different perspectives and you cannot get rid of them. But if I can write them down, or say "thank you" to my mind it helps to stop those thoughts.
And perhaps, and this is a trick that I do, when I talk to my wife, I will say something like "I feel that, and this is my mind, ...", so that she know that it is my anxiety talking and helps to defuse any tension in a conversation. Things can work out for you, but we each have to work out what works for us. If you want to chat some more....