It is a sign of spiritual health for you to feel guilty, sad and lonely at this time. This is the right barometer of your situation. The emotional attachment to your boyfriend can be compared to the story of the man that nestled a viper in his breast and cherished it until it bit him. As he lay dying he asked the viper, Why did you harm me when I only took care of you? The viper replied, It's my nature, I cannot help it.
Your soul is warning you that this relationship is harmful. However, it would take a simple act to change vipers into doves, namely, to bring this relationship into marriage in plain view.
If intentions are noble and acts mature, this possibility is closer and easier than you may think. If intentions are murky and self-centered, then you will continue to suffer under this burden because its constituents are betrayal as you said.
If you are already married and this is an extra-marital affair then you will need - after repentence - to avoid relations with your husband until after at least one menstrual cycle. If you are not married and decide to marry another person than your boyfriend then you have to wait the same duration. If you decide to marry your boyfriend then he also must repent. If he is non-Muslim he must accept Islam - which erases everything before it - otherwise marriage is forbidden.
Follow the advice of your Lord which is contained in the directives of the Law. This is the same advice you yourself would give to another sister if she were in your shoes. Win yourself long-term peace and stability rather than jeopardize it for short-lived, destructive passions. Your long-standing relationship with your Lord is more deserving of your love.Hajj Gibril