Im 23 years old and have been with my girlfriend for nearly 4 years and we own a house together. About 4 or 5 months I had a call from a female friend who I have known for many many years, and I have kept in touch via text messages and calls every so often. I have no feelings other than friendship for this girl and never have done. However for some reason I've hid the fact that I've been in contact with this girl from my girlfriend and don't know why, now she has found out about it and undestandably is not happy, and to make things worse rather than just come completely clean with her I've tried to keep the lies up... and this has made look like I'm guilty of something I'm not! I know I've done wrong and I have since stopped stopped contacting my friend. What am I supposed to say or do now to make things right with my girlfriend? I love her so much and have been through a lot with her and don't want to lose her, but I know I've seriously messed things up!

Be hoenst with her and tell her that there is nothing going on and that you are just friends. Admit that you should not have kept this aside from her as it does appear suspicious and show her how much you love her.
You have to keep in mind that you have to be honest with her and not lie again because once the trust is gone its hard to gain back

You need to just be honest. Remember that just keeping in touch with a girl that you weren't ever in a relationship other then friendship isn't wrong. Lying about it is.

You need to just sit her down and tell her the truth because I'm a girl and I know how I would feel if my boyfriend lied to me I would just want to know the truth. And tell her if she really doesn't believe you she can meet the girl so she won't get perinoid about it :)

Sadly, when you lie or try to cover something up, it understandably makes it seem as more is going on. Only you can figure out why you weren't honest about it from the start. Maybe you liked the attention from this girl, maybe you just didn't want to take the chance of your girlfriend getting upset. There had to be some reason, otherwise you wouldn't have felt the need to hide the communication in the first place.

You fix it now, but talking about it, acknowledging that you didn't handle it well, and reassuring your girlfriend of your love and commitment to her. It will take time, but your honest actions over time, is the only thing that will help your girlfriend to ease her concerns.

Tell her the truth, you were scared she wouldn't understand that the girl is JUST a friend.