Keeping your monster locked up and pretending that they aren't there never works. They will bang and clank their chains, they will slam against the bars, they will keep you up all night, they will shout awful things at you at the worst possible moment. In order to block them out entirely, you must go about in such a state of blindness, deafness, and denial that you will constantly miss things, a dissociating emotional zombie. You will also miss it when they saw through the floor and break out, long enough to make you say or do something you will regret later, before the internal guards lock them back up. You won't see them coming, and that's even worse...because then you have to make excuses for them and clean up their messes.
Given that this never works-even though most people have to try it anyway, usually for years and years- it means that sooner or later you have to haul your ass down there to the dark place and face them through the bars. Just finding the door may be difficult. Finding where you hid the key to the door may take even longer. Making your way, step, by step, down that dark stairway when you'd rather be almost anywhere else, that can be interminable. Still, it's better than being suddenly transported there without your consent, via depression or madness or pain or some other horrible happenstance. Go there on your own, while you're still healthy. It will be better in the long run.
Monsters that you throw in the same locked closet together often fuse. At the very least, they rub off on each other. That means that the things you are afraid of or disgusted by or angry about start to get linked together, often in weird ways. There's little hope of untangling them, at least not at the point where your monster is pissed at you.
This is an important point, so listen closely. YOU CANNOT DECIDE TO HEAL YOUR MONSTER. Let's say that again: You cannot decide to heal your monster. It won't work. While some people do manage to get their monsters to change, if you go down there with any hint of an agenda of changing them, they'll smell it. (After all, they do know you, they do live in your head.) They'll notice that you don't love them as is, warts and all, and they'll rebel and say "Fuck you." You'll get no cooperation out of them then.
That brings us to the most important rule of the lot: You must learn to love them as they are, horrid and dangerous and shameful and all. You must find it in your heart to love them, without ever hoping to change them. And no, you can't keep a tiny corner of your mind hopeful that by loving them as they are, they will eventually heal and change. First of all, they'll hear that, and they won't like it. Second, it might not happen. What if you were to go to your grave with them this way, still embarrassing and awful? You'd better find a way to be OK with that. The point of this is to get a better relationship with them, not to change them.
It's likely that this rule will be the hardest of all. It may take years for you to learn to love them, but it's necessary. It's the only way they will cooperate with you. Even if you wrestle them down, if they don't think that you value them as they are, they will fight you ever moment your back is turned.
That sad, sometimes you do have to wrestle them down. It's hard to do this in a spirit of love, but try to think of yourself as the alpha in the little pack of needs in your head, and when members of your tribe act uppity, you knock them back down and make them show throat to you. You don't hate them for it; you consider it part of the deal of being alpha. They're testing your strength, to make sure that you are strong enough to be trustworthy. And once you have wrestled your monster down, and he obeys- if only temporarily- he is no longer your enemy. He is now your dependent, and you must hunt for him and feed him. Taking care of his needs is now your responsiblity. You can decide which things he can have, and which are just not going to happen, period, but starving him is irresponsible behavior towards someone you want to trust you. He needs something to believe that you really value him.
When it comes time for negotiations, a starving monster will demand the Moon with ketchup on top. They are used to being denied, and they don't really believe that they're going to get fed this time-not after all the disappointments and denial and ignoring- so they come up with a truly ridiculous list of demands and claim that they won't be satisfied with anything less. Don't be fooled. Don't give up hope and go away in disgust, no matter how tempting it might be. Start small. Give your inner Beast some little thing that they want, or a mild version thereof. Yes, they'll want more, but you are the one in control. Give them as much as you can safely give them, then set the boundary. Over time, when they see that they are getting at least some of their needs fed, they will scale back on the demands. They'll still want those out-of-reach things, but they won't be constantly screaming for them. There are ways to compromise, but you have to make your way to the middle first.