What on earth are you on about?
You are 18, your sex life is your own business, not your mum's. You don't need her permission. Make your own decisions. If you need Mummy's permission, you aren't ready. But that would make you startlingly immature. Some people are, though, so go with what you are comfortable with. Just don't ask your mum to say "Its ok, dear, you have sex with him, I approve." That's a decision you have to make for yourself.
Trust you? To 'sleep over'? You mean, trust you to sleep in the same house as your six-month boyfriend without having sex? When you ask that you are asking your mother to rule over another adult's sexual activity - it isn't healthy and it isn't fair to her.
Your mum is free to rule that you will not bring a sexual partner into her home. That's fine. Keep to her rule.
If you are living with your mum and sleeping at your boyfriend's house will shame her in front of her community, don't do it. Both go elsewhere and keep your sex life private. No-one, not even me, will believe you if you say you spent the night with him and nothing happened.
Are you a Christian Fundamentalist? Or from another religion which is firmly against sex before marriage? If you are, keep to the rules of your faith, you'll feel better long-term. And get married. If you're not, do what you want and keep your personal life to yourself.
Ultimately, its your life, your body, your sex life. You have to do what you think is best. I prefer the 'fewer-lovers-the-better' route, but I've known perfectly nice women whom I have a lot of respect for who have taken an entirely different approach and don't seem to have suffered by it.
Your opening post seems to be saying 'How do I get my mum to tell me I can have sex?' Or worse, 'How can I pressurise my mum into being OK with me and my boyfriend having sex?' She doesn't have to do those things. You have to get on with your life and let her get on with hers.