Dear OP, I have also always had trouble feeling comfortable, safe and stable in relationships. I've been with my DP for 15 years now and it has been tough because I find the whole couply thing quite hard to sustain.
Looking back at my life, I think a lot has to do with the fact that I was the only child of divorced parents. I've been always very attentive to other people's feelings and needs, always tried very hard to please everyone.
This resulted in me being very needy and clingy in relationships, and in difficulties setting boundaries and protecting myself, so people tend to take me for granted. I was very scared of relationships because I couldn't face abandonement. So I had my first proper relatioship at 24 with my now husband.
I wouldn't say he's perfect but I always felt I could trust him, I could rely on him. He's a solid rock in my life, though I still have lots of ups and downs.
Maybe you need to look at your past. Where do those feelings come from? Why are you so anxious? There's always a reason why we are the way we are. We need to understand first why we react and feel in a particular way. Once you understand the why's, you can start to make changes in the future.
You also need to learn to set boundaries, and to make sure people understand from the beginning that you are not going to take abuse from them, whereas this is physical, emotional, or just silly day to day mind games.
Not everybody can afford to go to therary. I could never afford to complete a long treatment, which is what I probably need, but I read a lot about psychology, and the dynamics of human relations. Tbh, books have helped me a lot more than counselling.
I read a book that answered a lot of questions for me in terms of how to set safe boundaries and get to know yourself and what you want out of relationships and life. It's called:
Women who run with the wolves by Clarissa Pinkola Estes.
It was a great eye opener and it saved me millions in counselling.
I am a firm believer that the mistakes of the past can fully enrich your future, and that you can use all negative past experiences to avoid the same traps in the future. It's like cultivating a garden, you need waste and crap to fertilise the earth so as to make plants grow bigger, but you also need to do a lot of weeding, and keep always working on it :)