'She blatantly lied, but I can't come to confront her'

I wanted to surprise my girlfriend by meeting her at a mall where she said she was going to hang out with a few friends. But when I got there, all I saw was her sitting with another guy. I went away without her seeing me and when she got back, asked her about lunch. She said she had a great time with her friends. She lied blatantly and didn't even look like she was worried. I don't trust her anymore. I want to confront her, but I'm afraid this will damage our relationship completely. If I don't confront her though, I will never be able to look at her the same way again. What do you suggest I do?

It seems as if you have the answer to your question already. You can't let this fester because it will damage your relationship at some point, and you really need to tell her what you saw and give her a chance to explain. She may find it strange that you were, in effect, spying on her, so you may have to explain yourself carefully. There may be a perfectly logical explanation for what she did, so I suggest you don't jump the gun until you have a conversation with her about it. You are both adults, so have a grown-up chat and sort this out. And if it ends because of trust issues, then I'm sorry to say that it will be inevitable at some point anyway, because no relationship can survive without trust.

I happened to log on to my girlfriend's phone the other day and sent a friend of hers an innocent message on WhatsApp. She says I don't respect her privacy and has now protected her phone with a password. Is she overreacting?

Not really. It's her phone, her friends, and her right to decide who can and can't access her personal information. Put yourself in her shoes for a minute. I think you're the one overreacting.