I have an important presentation and a late exam today, so I decided yesterday to not fast today and did not wake up for suhoor. However, I feel guilty and was wondering if I could fast anyway (I have not eaten or drunk anything yet)?
First of all suhoor or eating before fajr is not a condition for a fast being valid or not.
However the intention is a condition due to the known hadith:
Narrated 'Umar bin Al-Khattab: I heard Allah's Messenger (ﷺ) saying, "The reward of deeds depends upon the intentions and every person will get the reward according to what he has intended. So whoever emigrated for worldly benefits or for a woman to marry, his emigration was for what he emigrated for." (Sahih al-Bukhari)
Therefore the Maliki, Shafi'i and Hanbali view on a person who intended to break his/her fast and wanted to revert his/her former intention -after the dawn of the day of fast- is that the fast is broken and the fast for this day must be made up.
The only case in which such a fast might not be broken is if the intention included a condition which was not accomplished.
If a person intends to break the fast when he is fasting, whether he is certain about that or is hesitant, then he does not find anything with which to break his fast and he changes his mind again, he has broken his fast and he has to make up that day. This is the view of the Maalikis and Hanbalis, but not the Hanafis and Shaafa’is. See: Badaa’i’ al-Sanaa’i’ (2/92); Haashiyat al-Dasooqi (1/528); al-Majmoo’ (6/313); Kashshaaf al-Qinaa’ (2/316).
The view that he invalidates his fast is the more correct view, as we shall see below. So if he intends to break his fast and is certain and not hesitant, then he cannot find anything with which to break his fast, so he changes his mind, he has still broken his fast, and he has to make up that day.
But if he is hesitant about breaking the fast, or he makes that conditional upon something, such as if he finds food or drink he will break his fast, then he does not find anything, then in that case his fast is still valid.
(Spurce: islamqa fatwa #95766)
However there's a small difference opinion within the Shafi'i madhhab as mentioned by imam an-Nawawi in his al-Majmo' -see here-, on whether the fast is broken right after one intends to break it or after a time in which eating or sexual intercourse is possible.
The Hanafi's however said (see here in Badai' as-Sana'ia):
ولو نوى الصائم الفطر ولم يحدث شيئا آخر سوى النية فصومه تام
And if the fasting person intends to break his fast, and nothing else occurs except the intention, then his fast is complete (valid)
And they use as an evidence the sahih hadith:
“Allah has ignored for my community what comes to their mind, so long as they do not act or pronounce words to that effect.” (Sunnan abi Dawod)
and they regarded a niyyah (intention) as something which came to mind as long as it was not related to an act.
As for the question part that you had no intention to fast on the night before this is a different topic and has different discussion.