What should i do ?

My mum doesnt trust me anymore.
I told this really big lie that she found out about.
And after that i realised what i did was wrong.
I was changing, i told myself i wasnt going to do it again.
But everytime my mum talks to me, she just cant seem to trust me anymore.
Before my sisters friends were over.
And she asked if there were any guys over, cause my mum doesnt like guys hanging at our house.
But he was standing outside with my sisters friend for the last 4 hours and when my mum asked i told her that there were no guys.
But then when she told me that if she found out there was a guy there shed be angry, i remembered there was.
So i told her, and now she thinks im lying again because i didnt tell her in the first place.
But im not. And she wont understand that.
She called before and she said she doesnt know what to do with me anymore and she has lost complete trust for me.
But this time i was telling the truth.
And i dont know how to make her realise that i wasnt lying, I tried to explain to her but she kept saying i was lying.

What should i do ?
I dont want to ruin the relationship between us..
Please help.

Be truthful from now on, your mum will start to trust you again because she will see that your not lying, when she sees this her trust will build up again.
when shes in a good mood tell her that you forgot about the guy, explain about him being outside and you hadn't noticed him, keep pushing until she gets the picture, and add that when you did remember you told her, this should help, make sure she knows that you didn't mean to tell her that there were no guys over, and say you didn't realise how important it is that she knows there was one.
good luck
hope this helps in some way. :)

Thank you
i do try to be truthful, but sometimes even though im trying its not good enough for her.
and ive already tried to explain to her that i forgot about him.
She wont listen because she thinks im only making up excuses.

I have to agree with Bob. Just because you have decided to change does not mean she is going to trust that or you until she is ready and you have proven yourself. If you don't want to be thought of as a liar, don't lie, a simple rule to remember.