While in that kind of situation, you need to do what you think is best for you. You should not teach yourself and yet you know that you do not like the other girl. You should give yourself some time to think then choose on one person.
are you limited somehow on dating only one person?..;-) do you and this girl have some sort of agreement? if not, i believe you are not betraying any trust issues. trust is very important in any relationship, and life! stay safe and enjoy your summer...;-)
Well take a break and don't date anyone for awhile and when you clear you head then you can decide what to do but the grass is not always greener on the other side of the hill. It is always good to be friends with someone before you decide to date or become intimate