... stress I had during the day.
Stress does have an associated sensation with it generally manifesting as a tightness in the head (as per Banthe Vimalasiri - see What is depression?) and / or lingering discomfort around the heart (as I have heard as being taught by other teachers to be consistent with the Abhidhamma as the mind is heart based). Based on the type of the sensation craving, aversion and / or ignorance would arise.
You have to relax both body and mind. The way to do this be equanimous of the session noting its arising and passing, or simply be with these sensation for a while analysing them before returning to your normal meditation object.
When you return to your normal meditation object have a mild smile. This is something you have is Goenka's meditation instruction though this is not emphasised much but Bhante Vimalaramsi emphasizes smiling a lot in his 6Rs formula.
Also one source of stress is worry which with you can counteract through concentration. When your mind wanders away realist it and bring it back. Periodically check if the mind is with the meditation object and even if it is fix your attention back at the object of meditation. Say if you are mindful of the body scanning part by part. You are looking at one part and the mind is still with it, then redirect the mind to the part again. If your mind is sticky stay with the part. If it is very restless either move from part to part quickly, or keep redirecting your mind to the object, i.e., fix your mind on the object, in quick succession. For someone who has practiced for a while this can break into tingling or electric sensation and further pratice this will become neutral. If you are moderately restless you can try Anapana meditation but if it is very restless best is quick scans as Vitarka dominates in this pratice. For a detailed study see: Saṅkhitta Dhamma Sutta, Uddhacca,kukkuca by Piya Tan.
... I noticed having ups and downs in my meditation, probably mostly caused by the amount of sati ...
Sati is useful to reduce though proliferation and eliminate Sankhara that follows. Make it pratice you review (paccavekkhana / vīmamsa) if you are mindful at defined intervals like end of each in or out breath cycles. This will help you strengthen your mindfullness. The description of how to review is found in Knowing and See, 4th edition by Ven. Pa Auk Sayadaw p. 35 onwards though is a slightly different context.
Sometimes it might be worthwhile to try a Samatha meditation subject in these situations.
... But I am also experiencing sittings, ...
Perhaps you can try walking meditation. Be mindful of the sensations due to body part movements and sensation of touch of the dress and / or air on your legs.
... where I am just overwhelmed by the hindrances and not even able to recognize them properly. What is the best way to deal with these extreme states of mind (in meditation)?
Extreme states are associated with extreme feeling. E.g. you feel extremely sleepy or restless. Be mindful of the feelings. If you want to learn / analyse how feelings are associated with the Hindrances in more detail, look at the Mental Factors corresponding to each Hindrance which is associated with each type of Mind State and the corresponding feeling of that Mind State. In short Ill Will be painful and the rest will be either neutral or pleasant but leave it upto the reader to figure this out exactly.
“Nothing is worth clinging to”
When this was said, the venerable Mahā Moggallāna said this to the Blessed One:
“In what way, bhante, in brief, is a monk freed through the destruction of craving, that is, one who has reached total perfection, the total security from bondage, the total holy life, the total consummation, the highest amongst gods and humans?”
“Here, Moggallāna, the monk has learned that nothing is worth clinging to.
And, Moggallāna, a monk has learned that nothing is worth clinging to, thus: he directly knows all things [he directly knows the nature of the all].
Having directly known the nature of all things, he fully understands all
Having fully understood all things, he knows whatever feelings there are, whether pleasant, painful or neither painful nor pleasant.
As regards to those feelings, [Section on Disillusionment and Revulsion (Nibbida) follows]
he dwells contemplating impermanence in them;
he dwells contemplating dispassion [fading away of lust] in them;
he dwells contemplating ending (of suffering) in them;
he dwells contemplating letting go (of defilements).
When he dwells contemplating impermanence in them, contemplating dispassion in them, contemplating ending in them, contemplating letting go, he does not cling to anything in the world.
Not clinging, he is not agitated; being not agitated, he himself surely attains nirvana.
The above is from: Pacala Sutta on Sloth and Torpor but can be generalised to the other hindrances. A general outline of how the 5 hindrances can be eliminated is found in Nīvarana,pahana Vagga and also following 6 essays Nīvaraṇa, Kāma-c,chanda, Vyāpāda, Thīna,middha, Uddhacca,kukkuca, Vicikicchā by Piya Tan.
Now coming to when you cannot meditate. Insight meditation is to experience things as they are. When you say you can't meditate, it looks like you are trying to do something during meditation. Just relax and stay with what you experience (reality as it is) to the best extent you can. Also there are many meditation subjects. Change of subject also might help.
Also have 2 sessions of meditation per day at the end of the day and beginning of the night (evening) and end of the night and beginning of the day (morning). If this case one session would probably be more productive than the other if you have a storm in one session.