My girl of 3yr (engaged for 4 months) just left me because she felt pressured into it and that she wasn't leaveing for another guy and need space. That was a week ago and I have had very little contact with her and today I ran into her at the store and she was with another guy and his arm was around her. When she saw me she knock it off and we talked for a min. I went around a corner and remembered what I needed to tell her and he was bring it back up. She was waer two things I bought her not to long ago. I am her first boyfriend. What do I do I really love her and she is the only one that I even look at

If she asked for space, then you should give her some space. I wouldn't think too much about the fact that she was wearing things that you gave her. When you look at the big picture that doesn't mean much. As hard as it might seem now, it's best to move on. It's not worth waiting around for someone that might not even come back to you. You'd just be wasting your time. She obviously isn't wasting hers.

Sit her down and have a proper talk with her in a place where you can both be alone.
You need to know the following things:
1) When she asked for a break, did she mean that she needed to think a few things over, or was it that she wanted time to get some more experience with other guys before settling down?
2) Does she have any intention of coming back to you, or was she just saying she needed a break so as not to hurt you're feelings?
3) Is she with the new guy?

You need answers, and it's not fair if she won't give them to you. While you may love her, you can't be expected to just wait around until she's finally ready to settle down. You need to speak to her at the end of the day, because that's how you'll get the answers you need.