Thank you for writing into Alterheros with your question.
You explain that you recently had an experience with a girl while still in a relationship with your boyfriend. After disclosing this to your boyfriend, he is willing to put this aside and to continue the relationship, but you feel as though you are more attracted to females. Now you are struggling to know whether you are a lesbian or bisexual.
First off, I would like to commend you on being truthful to your boyfriend by telling him about the experience you had with a girl. Sometimes it isn't easy to tell ones we love that we have cheated on them, but honesty and good communication can greatly benefit a relationship.
In your question, you wonder if you are lesbian or bisexual. Lesbians are women who are primarily attracted to other women. Bisexuals find themselves attracted to both men and women. Some people may even consider themselves as straight while still being attracted and even having experiences with someone of the same gender. This is why labels and categories can make life confusing! Sometimes it feels like you have to identify with one label or the other in order to feel comfortable with who you are. In this sense, labels can help us organize our identities and help navigate through our social interactions. However, it is entirely possible that you can be perfectly happy without adhering to a particular word to describe your sexual preferences. Some people may find themselves attracted to the personality of someone without thinking about what gender (s)he may be. It is up to you whether or not you wish to define yourself with one term or the other.
Let your experiences and feelings guide your sense of self and your sexuality. Be true to yourself and honest with those around you. This is a time of discovering who you are. If you feel comfortable using a particular word to describe yourself, then embrace it. At the same time, try not to be too burdened by finding the right word to identify yourself if you are unsure or reluctant to label yourself.
Remember, you can do what ever you want to do and be with whomever you want to be with. The important thing is to feel comfortable in your own skin!