Hi, I'd like to help you with your question.
It sounds like there may be something from your past that interferes with how you see your relationships. It could be how your parents interacted (possibly a dysfunctional relationship?) or a bad experience, etc. Whatever it may be, it could be interfering with you enjoying your relationship as it is.
Just the fact that you are aware of how you are reacting to this relationship is good thing. Your insight will help you avoid reacting the same way you did before in previous relationships. Explore what you feel may be at the root of your fear. Look at how it may interfere and be aware of it. If you feel it warrants therapy, consider seeing someone. However, just being aware may be good enough.
Explain how you feel to your boyfriend. Let him know it's not his fault and that you are working on it. This is so he realizes it is not him or the relationship that is bothering you. It makes it easier for the relationship to continue.
Take the relationship slow. There is no reason to rush, so see where it goes and allow yourself to feel each step of the way. If you feel you are started to feel overwhelmed or your feelings are interfering in the progression of the relationship, see a therapist to help you work it out.
Here is a link to help you find a therapist:
I hope this has helped you,Kate
If you already have anxiety and are also having flashbacks to another similar relationship, then feeling the way you do would be natural.
Try practicing some relaxation exercises. Also, you can work on the anxiety itself to help you feel more in control. Here are some resources that will help:
The Anxiety & Phobia Workbook (New Harbinger Self-Help Workbook) by Edmund J. Bourne
The Cognitive Behavioral Workbook for Anxiety: A Step-by-Step Program by William J. Knaus
Both of these books have numerous ideas and techniques you can use to help calm your anxieties and work towards feeling better. Kate