Throughout your message I received the impression that the relationship lacks a firm grounding, and is moving far too quickly under the present circumstances. From top to bottom, here are a few comments:
Over the years we both lost trust and confidence in opposite sex, but we were desparate each for their own reasons, and we both hoped there is someone out there waiting for us.
When we met she fell in love in me and asked me out. On the first day she explained her situation and told she would be crushed if I'd hurt her too.
This immediately raised a red flag. I don't believe there's anything wrong with seeking compansionship or wanting to end loneliness or pain, however rushing into a relationship seems to me like only a way to increase that pain in the future. While falling in love at first sight is possible, I believe you were right in being somewhat cautious of her.
She said she had all the time in the world becouse she was sure I was the one she was waiting for.
After that amount of time, this deserves another red flag. It takes time to get to know a person, a statement like that generally implies extensive knowledge and trust.
When she came back, she said she really didn't mean to, but she did cheat on me with some much older (29) guy (kissing and hanging out with him).
Now she is not sure about feelings towards me or towards that guy. She said that this guy is not an option for her becouse he is much too old for her, but now she is unsure about what to do with me.
Disloyalty and uncommitment. Pardon the frankness, but if you stated that you had found the person you were wanting to spend the rest of your life with, could you bring yourself to cheat on them, especially after only a few months? She should be thankful that you didn't end the relationship at this point -- I certainly would have.
I don't know whether it was just your wording or was actually her own, however "what to do with you," doesn't strike me as an acceptable comment for a mutual relationship.
I had a realy long talk with her about that and I found out that she was attracted to him becouse he was much more "free" with her in public and privately.
Being open and sincere with a partner can usually help to sustain a relationship, but when it takes such a little gesture from a stranger to cause her to betray your trust, I would develop serious concerns about her trustworthiness. Has she ever come to speak with you about her concerns regarding closeness and trust? If not, why?
Am I too possesive towards her? Was this kiss a mistake? I only wanted to show her that I can be "free" with her. Am I putting her under too much pressure?
I told her I want something special with her and not only an affair. I also mentioned marrige some time in a distant future. Did this scare her?
I wouldn't consider you posessive for wanting her to be commited and trustworthy. Instead of showing that you can be open with her by way of a kiss, would she be interested in a private and personal conversation to help break through some of the ice?
You are right in wanting more than an affair, however I'm not certain as to whether she is genuinely willing to participate in anything more than that, with whichever label to her actions that she would choose to give.
I trust her, and I belive it was only kissing with a guy she met. Nothing more. But I can't understand how someone can be absolutely in love with someone one day and the day after forgets everything.
A kiss with another man is not something to dismiss on a whim. In a relationship, that is not acceptable, regardless of the circumstances.
In my previous relationship, I recall cringing when hearing, "I love you." Unfortunately, while the words might be said, the true strength and support behind them may not exist. That was the case with my relationship, as it may be with your own.
I know I am emotionaly not able to cheat on someone I am involved with.
I agree with you entirely on this comment. I could never bring myself to cheat in a relationship, and would rather end the relationship out of respect than betray the trust that it commands.
Although I believe the relationship was started with the best of intentions, from your description, it doesn't seem to me that she is ready nor able to participate in a commited relationship, and I would be very dubious of her actions, regardless. From a personal standpoint, after such a short amount of time in the relationship, her cheating with another man, even if it was only a kiss, would cause me to instantly end the relationship. I respect that you care about her, but I don't believe it's a good idea to continue the relationship at this time. At the very least, I would suggest that you give her plenty of time to sort out her mixed feelings and decide which path she would like to take.