I'm afraid to start "bulking""(building muscle mass) because I might put on a lot of fat — what should I do?

Where you remain has to do with your self-image. Your fear tells me you are still not comfortable being lean ... so that is certainly something to focus on. You want to create the image of the healthy and body you deserve, and remind yourself each and every day. There is a saying I've heard from Fred Lehrman that goes ... "Believe, but don't believe your beliefs."

Sounds odd, but we are creatures of habit and programming. If you don't have the concept of you living healthy and lean, create the action and make it a "forced habit" that you consciously decide to do each day. With time, like your other programming (i.e. the programming that caused you to gain weight) you can create a new program - a new belief, if you will - that you are lean and your subconscious will be your best friend with keeping it that way.

To suddenly add even 1 pound of fat, you would have to over-eat 3500 calories. Now, even for former binge eaters like myself, that is quite a feat to achieve. When you embrace this as a lifestyle, you realize you don't have to fear. You can slowly add calories and slowly add weight, and then if you find you are gaining too quickly, trim back before you go too far.

The fear is really not fear of bulking or adding fat. It's fear of letting go and resorting to your former self - of backsliding. So focus on what it truly is, and remind yourself of your power and brilliance, and step into who you deserve to be and most importantly, get comfortable with you.
I don't know too many people who have to beat themselves up to take a shower or brush their teeth - these are patterns they've created and do almost subconsciously. If you continue to focus on the new you, you can create a subconscious habit of living healthy and being in shape, too, and all you do will move towards that end rather than serving a self-defeating purpose.