illustration (attribution, if any possible, is at the end of the article)
Cultivation of concentration and mindfulness allows for some sort of control of your mind. You learn how to drop thoughts (hopefully to drop them fast but not too fast: koan.mu/be_present.htm ) hence you learn to prevent mental proliferations.
By letting go of mental loops, you may feel "in control": if you can avoid perpetuating e.g. sadness, you may start conceiving yourself as able to change sadness in something else. This 'control' is an illusion.
The key to freedom is in not appropriating thoughts, or feelings, as 'yours'; this is not the same as controlling them.
It doesn't matter what the thought is, or what the feeling is: if you can refrain from identifying with it, you are free from it. How you respond becomes a choice.
Once you appropriated it, you "have to" respond (in a way consistent with other appropriations vis-à-vis "who you are") because you "have to" live… but you didn't need to appropriate it in the first place!
Grief is a classic example: "the universe has lost a wonderful person" might well be valid, without you having to appropriate it as " I have lost a wonderful person." Sadness may exist without you making it yours. One way to see this is that the said sadness might be shared with others: it is not 'yours'.
The ability to cease mental proliferations gives the ability to get out of their bondage, of their conditioning of our next response, of karma. However, it does not give the ability to transform one thought into some other thought.
Learn freedom, not control!
The idea that bliss is achievable by controlling the situation at hand is the very illusion of saṃsāra, of the rat race, of the pursuit of happiness by arranging external factors into a desired configuration (which will by nature prove to be impermanent hence unsatisfactory). Learn freedom, not control!
Don't try not to be sad. Only refrain from identifying with sadness, refrain from consuming all your existence into sadness.
Freedom is attained by not letting sadness decide your life for you, by not letting sadness bias how you'll respond to the situation: when someone dies, the compassionate, loving, patient, perseverant, wholesome, wise answer is often found in caring for the 'living' left behind… Freedom is in not letting sadness over one death blind you from the many still alive (including yourself). Honouring the lost ones often takes the form of continuing what good they contributed, rather than lamenting that there's no one else to pick up the tab.
Then the Venerable Ananda went into the vihara and leaned against the doorpost and wept: "I am still but a learner, and still have to strive for my own perfection. But, alas, my Master, who was so compassionate towards me, is about to pass away!"
And the Blessed One spoke to the bhikkhus, saying: "Where, bhikkhus, is Ananda?"
"The Venerable Ananda, Lord, has gone into the vihara and there stands leaning against the door post and weeping: 'I am still but a learner, and still have to strive for my own perfection. But, alas, my Master, who was so compassionate towards me, is about to pass away!'"
Then the Blessed One asked a certain bhikkhu to bring the Venerable Ananda to him, saying: "Go, bhikkhu, and say to Ananda, 'Friend Ananda, the Master calls you'."
"So be it, Lord." And that bhikkhu went and spoke to the Venerable Ananda as the Blessed One had asked him to. And the Venerable Ananda went to the Blessed One, bowed down to him, and sat down on one side.
Then the Blessed One spoke to the Venerable Ananda, saying: "Enough, Ananda! Do not grieve, do not lament! For have I not taught from the very beginning that with all that is dear and beloved there must be change, separation, and severance? Of that which is born, come into being, compounded, and subject to decay, how can one say: 'May it not come to dissolution!'? There can be no such state of things. Now for a long time, Ananda, you have served the Tathāgata with loving-kindness in deed, word, and thought, graciously, pleasantly, with a whole heart and beyond measure. Great good have you gathered, Ananda! Now you should put forth energy, and soon you too will be free from the taints."
— Maha-parinibbana sutta (DN 16)
Learn freedom, not control: as a first step, learn how to meditate
Acrylic painting: "Clearance", © Nicole Roumelioti