Feeling paranoid and insecure since I got back with my boyfriend help?

At the start of May my boyfriend of just under a year dumped me because he wanted to travel to Spain for 6 months in September to do EVS (european vocational scheme) which left me heartbroken and feeling siucidal. Within a week and a half he came up to see me and told me his life is hell without me and that he'd ripped up the application forms for EVS. I decided to give him a second chance, but told him he'd have to be patient because he needed to build back up my trust and to get me back to what was once my normal happy self. Since then our relationship has being going well, but I just have this huge fear that I'll get my heart broken again. I've been having nightmares almost every night since he dumped me, a lot of them about him breaking my heart again. And although he always compliments me and treats me right, my self esteem is lower than ever. I'm living in fear of him finding someone or something in life better than me, and I find myself crying a lot. No matter how much he reassures me, it doesn't help. It's not him thats the problem, it's my self esteem, fears, and insecurites. What can I do?