From what I understand from your question, you have recently developed feelings for your friend. This is something that you haven't experienced before, as you identify as straight. You are struggling with these feelings because you are worried that it may have a negative impact on your friendship.
First off, I would like to assure you that it's not uncommon to find yourself being somewhat attracted to a close friend. Sometimes it turns out to be a friend of the same sex! It is courageous to own these feelings and seek out some advice about them.
It is very important to be honest with your friend and yourself. Telling your friend that you have feelings for her may be one of the ways to untangle these confusing thoughts for you. Even though it may be a bit awkward and uncomfortable to do, this will pay off if you can get your feelings out in the open.
It is understandable that you do not want to harm your friendship. Hopefully, revealing your feelings will not have a negative impact on your friendship. Keep in mind that your friend may react with shock or disbelief–especially if she knows you identify as straight. Allow her some time to come to terms with this new information about you. On the other hand, she may welcome your revelation and help you through the difficult process of understanding your feelings.
The way in which you go about sharing this information is up to you. It may be a good idea to ease her into the idea that you have an attraction for girls well before confessing your feelings for her. She may be caught off guard by this information, so taking baby steps' may help her adjust. Since you are very considerate and do not want to use your friend to experiment with your feelings, approach the subject with your friend in a respectful way and remind her that you are unsure if you really are attracted to girls or not. She will hopefully empathize with the situation you are going through and will help you through it.
If you are feeling uncomfortable about approaching your friend about this matter right now, you can always talk with someone you can trust in the meantime, if you just need to vent or get your feelings out into the open.
Thank you for coming to AlterHeros with your question, and please come back again if you have any future questions.