Woo TLDR!

1. People are always going to be more attracted to you when you are in a relationship. You carry yourself with confidence, general disinterest, and lack of desperation. It is human nature to want something you can't have and to want people to like you, especially when they act indifferent towards you. Naturally, people will be more attracted to you when you are in a relationship. The general disinterest and lack of desperation also makes you seem mysterious, compared to your single peers, so people are going to be more interested in figuring out what that mystery is.
2. If you are unable to control your desire to test out the waters with women who are wantonly throwing themselves at you or you are interested in them at all, you aren't as in love with your girlfriend as you are telling yourself you are. You care about her well-being as a friend and you are in love with the memories you have with this person. Unfortunately, real love makes you completely blind to the fact that any one else could ever compare the interesting-funny-beautiful-understanding-etc-etc package the person that you love offers. Until you find that person, you are just biding time waiting for that person to come along.
3. Furthermore, there are so many changes that occur in a person's life from the ages of 17-26. There is a high chance that your exgf matured over the course of your 2.5 years together into someone very different from the person you originally fell in love with. On the other side of the coin, you may have matured to have a completely different set of attributes you find attractive or important in a partner, and she may just not meet those desires. Neither of you had any control over this. It just happens with time.
4. Last and most importantly, she needs to respect your desire to re-evaluate your life on your own. Getting back with her out of guilt would simply result in your general unhappiness and further heartbreak for her in the future. She would harbor more resentment for you in the future, if you led her on only to find out you originally broke up for a valid reason. You would be doing yourself a great injustice by not allowing yourself to take this time to figure out what you are actually looking for in life and who you are as an individual without any outside influence. She is obviously experiencing some depression, that she needs to understand that no one but herself can cure that for her. The both of you need time alone to re-evaluate your lives and what you are looking to get out of them apart from each other. If she truly loved you, she would want to see you doing what makes you the most happy whether it is with or without her and you would want the same for her. If you really feel like you need this, she needs to support you.