There will be days where all you will feel is sadness. That is OK. You are allowed to just have days where you feel off and want nothing more to just curl up in a blanket. However, this doesn't mean that you should treat that as an everyday feeling.
Taking accountability of your mental health sounds hard and might make you feel like you are your own worst enemy, but it will be worth it in the long run.
What I mean by this is very simple — every day, do a mood check with yourself. You keep count of how many days you feel the same and at the end of the week, you try to come up with a list of reasons why you felt that way and if there is room to improve your mood for the next week. If you felt happy all week, then that is fantastic. That just means that you shouldn't find reasons to change your mood for the coming week but you should think about why you felt happy that week and how you can carry that energy into your other weeks.
It's hard to hold yourself accountable for anything. That's why I also recommend having an accountability partner. If you have a friend or a family member you trust, then have them check on you at the end of each week. If you can't find the time to do it yourself, this will be the only way that you can guarantee having good mental health.
It is challenging to seriously take care of your mental health — I struggle with this myself. I tend to put others' mental health before mine and it ends up ruining me later in life. I have a long way to go to get my mental health back to the way it used to be. My friends all tell me that at the end of the day, the only person that will be my biggest supporter and friend is myself. I need to take care of myself first THEN worry about others' happiness. I sound like a hypocrite writing this because to this day, I still can't put myself before others and it's hurting me quite a bit.
Standing up for your own happiness should always be your number one priority in life. That is why holding yourself accountable for your mental health is super important.