>I don't want this relationship anymore. Am I overacting?

Why would you think you're overreacting... your judgment is sound:

• She has been rude, cold and distant with me
• Giving me the silent treatment for 3 months
• she didn't celebrate my birthday
• didn't celebrate me getting my bachelor degree and getting a good job.
• Sometimes she gets mad over little things because she got borderline personality disorder

Taking the 'cheating theory' out of the equation, you're left with the above.

That is enough of a reason to end a relationship.

She already stuck you with the knife... the cheating theory is just her twisting it.

Follow through with this:

>I don't want this relationship anymore.

This relationship doesn't sound like its worth being in. Break up, take some time for yourself, and find someone who can offer a healthy relationship and make you feel loved.