A lot of problems here.

1. Your LTR was messaged by her ex. This means the break wasn't clean, clear, obvious, and eternal. This isn't necessarily her fault. A lot of guys will try to hook up with an ex, and maybe this guy really did contact her out of the blue and unsolicited. But this does show that she left the door open a crack when she ended things with him.

2. She hid it instead of telling you. This means that she considered her ex-boyfriend's offer. It also means that she knows you're a little bitch who would get all whiny and jealous over it. In a better situation, you and your girlfriend ought to be laughing together about the texts she gets from other guys. How did you find out about this if she didn't tell you? Do you habitually go through her phone? If you have to do that, you, and your relationship, have serious problems.

3. She told you she replied with 'I have a boyfriend'. Do you seriously think those four words were the complete extent of everything she texted in their exchange? Of course not. She's keeping the details from you. Maybe she has something to hide, maybe it's because she knows you'll be a little whiny bitch, maybe both.

4. You don't know if you should believe her or not. Deep down inside, you know she doesn't respect you. She thinks you're a whiny bitch. That's why she hides things from you to avoid dealing with your crap. Because she doesn't respect you, you know that your relationship is temporary and any day, she might get a text from a better guy and be gone. She hides things from you, you know it, she knows it, and it's mostly your fault.

5. She deletes text messages. This is ONLY DONE WHEN A WOMAN WANTS TO HIDE THINGS. When you asked her about it, she told you, flat-out, that you are a whiny insecure bitch and she deleted her messages because she didn't want to deal with your crap if you saw them. Do you go through her phone? And she knows you do so she deletes things she doesn't want you to see? That's some bad crap.

6. You claim you've never had an issue when she gets messages from guys, yet here she is acting like you have 1000 years of history of being a whiny bitch. Maybe you're telling the truth. I don't know. But whatever the truth is, she clearly thinks you're an insecure little bitch, even if that's completely false. No telling why she thinks that, but she does. So she's going to act that way in your relationship.

7. You don't know what to do about this. You can feel it in your bones. If you really stop and let yourself think, you'll recognize that you consciously know it and have been denying it: Your girlfriend doesn't respect you and your relationship is on its last legs. It's only a matter of time before she finds someone better and dumps you. So what you should be doing is meeting your girlfriend once a week for sex, while going out and doing your own thing the rest of the week and meeting other girls. Maybe you'll find someone better and replace her.

Keep working on yourself. Hit the gym, eat well, kick ass at work or school or whatever it is you do, and stop expecting your shitty girlfriend to act like a good wife. Let her be a shitty girlfriend, keep fucking her for however long it lasts, and have the next girl ready for whenever you're ready to move on.