Relationship anxiety

Just hoping to get some advise on dealing with paranoia/insecurity/anxiety in a relationship. I have felt insecure most of my life but it is crippling my life now. It really strained my last relationship and I don't know how I am going to get on and be happy again. Basically when I was with them I was constantly insecure. Every single day I would imagine scenarios over and over where they were cheating on me. It caused me so much pain and made me feel depressed. After the lockdown it got worse. I was crippled by fear, maybe separation anxiety too. I Feared the moment I left they'd invite someone over and do the deed. I was constantly checking their phone/computer for evidence. It was toxic. I hated myself for it but I didn't know how to deal with it. They had left someone else to be with me but a few things happened which I'd rather not say which did cause problems between us. The thing is, they still want us to be together in the future and I just don't know what to do. I don't want to have those feelings running rampant in my mind and controlling my life. I need to deal with this issue first. I need to know what I can do to control it. I am very defensive and always have a wall up because of fearing rejection. A part of me thinks well they were with me all this time and nothing happened so why not trust them? But then my cynical side says 'what and then them cheat later on down the line.' They had tried hard to reassure me a few times but it was no use. I just wish I didn't care so much and thought 'just live life and enjoy the time you have with them.' But it seems impossible. Any advise would be appreciated. Thanks.