It becomes easier to do once you build up enough awareness and intuition for the mechanism. You can take up arbitrary frames of mind with a little imagination, especially things you've probably already experienced before (like your previous enjoyment of your hobbies). No matter how stuck you are in a particular frame, you can always call up another one, at least for a moment. Professional actors might or might not be aware of this particular mechanism, where they can call up a frame of mind that compels laughter or crying or anger, rather than consciously guiding their actions that often results in fake and unnatural acting. A good actor, if he needs to get angry, will intuitively enter a frame in which some grave injustice has been done to him by the target of his anger, then the angry behavior becomes compelling and natural. You've probably heard of method acting as well, where the actor actually spends months preparing for a role in order build enough experience and intuition for the kinds of mindsets and behavior required.
The first thing to understand is that these frames of mind are fundamentally instrumental in nature, they serve a purpose and you can get stuck in certain frames of mind if your current present situation calls for it. If taking up the particular frame of mind offers emotional stability, you will tend to stay there unless you find a better frame or you realize that it no longer serves its original purpose. A depressive frame which makes depressive symptoms compelling i.e. psychomotor retardation, demotivation, lack of pleasure and so on, can itself be useful to the person and the person might actively keep themselves in that frame, usually unconsciously (or even consciously with the LDAR philosophy).
Consider someone that's in a bad situation and they have to deal with all kinds of negative emotions like uncertainty, fear of failure, anxiety, being overwhelmed etc. A certain amount of people will end up in a depressive frame as a way of coping. Either they arrive at it intuitively or they learn it by observing other people, especially family members, like a mother that takes up the depressive frame every other week and lies in bed for days. Once you learn to take up that frame, you will tend towards stability and when your mind is faced with certain unmanageable emotions, you retreat into the safety and comfort of that frame even though it might bring its own undesirable features.
Seeing potential in things and having hope can be something undesirable, as potential can often bring about anxiety, a sense of uncertainty, feeling overwhelmed, creating the possibility of failure or embarrassment and so on. If you don't see any potential in things, then you won't feel any pleasure or look forward to anything, but you might be in a more psychologically stable state.
Another common "use" for depressive frames might be the attention you get or believe you will get if other people see you down in the dumps. This is usually more common with females due to how society often affords them lots of emotional support from a young age. A crying boy will probably be told to rub some dirt in it and go back out there, while a succubus is much more pampered. Gradually she learns quite implicitly that this frame of mind brings with it something rewarding and bouts of loneliness or other difficult emotions might act as a trigger for her to take up the frame and find some stability and comfort. A seemingly non-sensible behavior like self-harm makes perfect sense from that frame of mind, the more I suffer the more people will care (this is not a conscious thought or calculated thing usually, but just a feeling or sense of anticipation that can be pleasurable by itself).
To summarize, shifting from one frame to another is firstly, a matter of self-awareness, running into this mechanism and becoming acquittanced with it from first-hand experience, actually observing the lines between frames and the shifts that happen. Secondly, some amount of knowledge (previous experience) and/or intuition is required for the new frame in order to be able to grasp it. Thirdly, it requires that the new frame be a stable and fundamentally useful one to you, otherwise you revert back into something more stable. An actor goes back to being himself after the camera stops recording, a depressive person momentarily finds himself looking forward to something only to be reminded of some emotional pain and he retreats back into grayscale land where things are stable. A lot of wizzies have these bouts of "I can do it!" until they eventually retreat back into the "woe is me" – building awareness of this is the best way to deal with it. During my own dealings with depression, at some point I could notice myself going into those frames of mind and noticing is half the battle. These days, I can still enter a depressive frame but nothing holds me there and illusory comfort of that frame doesn't last long for me and I quickly go back to something neutral.
"Decades of conditioning" doesn't matter, a frame you've inhabited for years can be shifted in a moment and you will be able to stay there as long as the new frame is something you actually want to take up and doesn't just invite emotional pain or instability. If you can understand how a depressive frame can be fundamentally useful and comforting (despite its negative features), you can start to observe what exactly keeps you there, how exactly does it afford you stability or how it might compare to the new frame you want to inhabit.
>Can you just tell yourself you enjoy a particular meal when you don't?
As long as the food is palatable and not overwhelmingly disgusting, you can probably take up a frame of mind that finds it enjoyable. What "enjoyable" means depends on the exact frame as the pleasure is contained in an anticipation of a reward, rather then direct sensual experience. The example of self-harm above, is that it's sensually painful, yet emotionally pleasurable. Other inherently painful things can also be enjoyed, like weight lifting or manual labor, as long as it's done with a certain frame of mind which anticipates a reward from it. The majority of things people enjoy are things they learned through experience are rewarding and don't directly provide any kind of sensual pleasure i.e. enjoying reading has nothing to do with enjoying how your eyes move back and forth and holding the book itself (those might actually detract from the pleasure), but rather an anticipation of reward that could be described as curiosity or what the content of the book might afford you or what reading the book means about you etc.