No one should ever be afraid in their relationship with their intimate partner. Period. However sweet he can be and however much you love him, you are wise to question whether you should sign on for a lifetime of unpredictable outbursts and fear. Having bipolar illness is not a reason to leave him but his refusal to get treatment certainly is. You met this fellow when you were only 16. Although you’ve been together for 4 years, those years have been an important time for you both of you to mature. Apparently you did and he didn’t. The reason you find yourself looking around is that deep inside you know that you can’t take his behavior over the very long run of marriage. Trust yourself. Go back to your therapist and work on that some more. You deserve better. (By the way: So does he. He deserves to have a life that is more manageable and where people who love him aren’t afraid of him. I hope he comes to his senses and gets good treatment before he does something he will always regret.)
I wish you well.