I can understand how you feel about being protective of the ones you love the most. Sometimes this protective feeling plays a bigger part of life than you would like.

You want the best for your mum and want her to be happy. The one thing that we all want for our loved ones.

Can you talk honestly with your mum about what is going on? Open up to her and let her know how you feel, and encourage her to be honest with you on how she feels about you and her friend. Talking is the best way for finding out the truth and get to know the feelings involved.

Talk and let us know what happens.

Tony's right....Talk with your mom and let her know how you feel. Maybe the guy's just taking a breather....And once he knows your cool with the relationship he'll make an appearance. If not.....He probably wasn't the right one anyway.

And by the way....Something you'll learn as you get older and wiser....The job doesn't make the person....It's their kindness...The person who just by knowing makes you feel like a better person. :)

Hi there hun. Don't blame yourself for this, i'm sure it is not all down to you. People make decisions for all kinds of reasons, partly to do with what is going on for them not just other people's pressure or reactions. I'm sure it is not all down to you, if at all.
Your mum maybe will manage being "alone" better than what you anticipate, and she has you so she is not totally alone anyway!

It is only natural to feel defensive and anxious about new people coming into our lives especially when it is beyond our control and when it is sharing a house with someone new!! That is a big step for anyone to take and it is only natural it has worried you somewhat. Don't feel guilty, it is your emotions natural defenses acting out.

I agree with the others about talking to your mum about how you feel. Make it clear you really want her to be happy but you feel a little afraid and worried because you don't know this guy like your mum does, and you need to feel safe and comfortable too as well as she does. And also explain you are worried for her to, that she may get hurt etc. (you can do the same if a new man comes along etc).
Don't worry, your reactions are very common and only natural, you're not being selfish or nasty. (i work with teenagers and come across these types of situations a lot!)
good luck, let us know how you get on.
Loops x