Mum has threatened to leave me

Charlie asks :

Hello Lucy, I've got quite a big problem with lots of little problems intertwined. My boyfriends of 2 years and my best friend for 10, has always had up and down periods with my mum. When they get on, it's like they're closer then me, but when they're not its absolute hell for me. My boyfriend has no family and not many friends, and is now going through the processes of being diagnosed with bipolar and a few other mental health issues, and despite everything I'm about to tell you I genuinely believe he is the love of my life. My mum met her current boyfriend online, and has been dating him for about 8 months. They are madly in love and spend every weekend together as he lives an hour away, so she only ever goes there. I'd like to add at this point I've only met him twice. My boyfriend, having been on his own from the age of 13, got involved with a bad way of life, which I'm now pleased to say he is out of, but certain things still crop up from time to time from his past that are sometimes tricky to deal with. The most recent was an issue of money, from somebody just out of prison. I was unaware of this. My boyfriend and my mum had just made up from on ongoing spat that lasted around 4 months. Instead of asking me to lend him the money, he took a card he assumed to be mine but in fact belonged to my mother (same pin) and helped himself to £60. Supposedly he instantly regretted it but was too embarrassed to say anything so decided to wait until he had the money to pay it back. Meanwhile, obviously my mum has noticed. In an instant she's thrown all of his things in a black bag, sent him an abusive text and changed the locks. Understandably. She then said I could have nothing more to do with him at all. It broke my heart. I didn't know what the right thing to do was. Although she's only been with her boyfriend for a short amount of time, her plan was/is to move up to his house next year, leaving me paying rent to her in our current house, so I would still have somewhere to live. After meeting my boyfriend I knew I couldn't just let it all end, there must be some way to fix it. Me and my mum have been on our own for some time now and at one point not long ago was my best friend. I found my self sneaking around and lying to her so I could meet my boyfriend. It breaks my heart. She's not stupid as suspected this, and told me if she ever caught me speaking to him she would sell the house and move to her boyfriends and leave me, that's how strongly she feels about it. How am I supposed to choose between them? They're all I have. And my mum is eventually going to leave me alone anyway, so what difference will it make? I feel lonely as it is, let alone without one of them. Things between me and my mum are now awkward and she's made it pretty clear she doesn't want to talk to me. This has all happened in the space of a week and here we are. I'd also like to point out I'm 19 and still feel very much like a child. Please help me in any way you can, and sorry that it's such a long message.