You need to have a very frank and explicit conversation with your boyfriend about your mental illnesses. He may have an idea that you have some problems, but from what I’ve gathered, he has no idea of the enormity of it all, partly because you have purposely tried to shield him from it.
So first, you need to have this talk with him. He will probably not respond in a way that you like. He might try to say that you are just being manipulative. He might just try to shut you down and tell you to knock it off. Or you might get lucky and he will see the light and want to help you (or walk away if he can’t handle it).
Next, you need to find a competent therapist as soon as possible. Talk to your primary care physican (not sure how that all works in England) and have them refer you to a psychiatrist. You have multiple problems and it’s going to be tougher to treat your illnesses, but the problems that you have cannot be solved on your own, they cannot be solved by simply staying with your boyfriend and hoping for the best (your jealousy issues alone will most likely be the reason that your boyfriend ultimately leaves you, and this will be true whether or not he is cheating).
If, after your conversation with your boyfriend goes well, and he lets you know that he loves you (you need to ask him that question, directly, since he hasn’t actually told you that he loves you, because it’s important for you to know, one way or the other, or whether he’s just staying with you because it’s easier).
If he lets you know, in no uncertain terms that he really does love you and wants to help you, then you have to let him in on exactly how this is going to go from here on out. You need a doctor, you probably need lifelong medications (which may have to be figured out by trying one, then another, then another, then another until you get the right combination) and you will most likely need ongoing therapy. You should probably also consider going into couples counseling with him. He clearly does not know the extent of your mental illness and it’s not fair for him to just continue to live with you (while you hide the information about your illness from him, while at the same time snooping on him constantly). He needs to know exactly who he’s dealing with, and you need to know exactly how much help he is willing and able to give you, and whether it’s worth it for you two to stay together. At this point, you don’t seem well suited towards each other.
You need to reach out to other people (family, people that you know, people in support groups that deal with the illnesses that you have) and you need to do a lot of research to find out what might actually be useful for you, and not just poo-poo the information, otherwise, you’ll keep going round and round on this same treadmill.
This isn’t going to be easy, but the alternative is that you will simply continue to be sick, you will continue to accuse your boyfriend of things (that he may or may not be doing) and he will eventually get fed up with it (whether or not he is actually cheating or thinking about getting back with his ex) because people don’t like to be accused of things (whether or not they are guilty of doing those things). But if you just clam up, then this situation will stay the same for awhile, then eventually get worse.
Today, you have to make the decision to change the way things are going, with yourself, with your boyfriend, how you make decisions, how you share information, how you gather information, and what is important with regards to you living a healthy life (with or without your boyfriend).
Get out a piece of paper and actually write down the steps that you need to take. Because you have so much anxiety, if you don’t write this stuff down, then you will forget what you need to do and delve back into the same routines. Your boyfriend also needs to know exactly what you need to do to get help, so if it is written down, it will be easier for both of you to remember what needs to be done, and start going down the list, one by one. Starting with you having this conversation with your boyfriend, then contacting your doctor and so on and so forth.
Here is some information about getting help from the National Health Service in England
Here is a site for Self Help Groups for BPD in England
Here is some more information about Getting Help for BPD in England.
And still More Info about getting help for mental illness in the U.K.
Good luck to you, dear.