No Muslim can ever guarantee that Allah has forgiven him. To say this, you are basically a person of Jannah, and that’s not something that any Muslim should say. But yes, in the sense, you should expect and hope.
There is a big difference between hoping and guaranteeing. You should hope and expect that Inshallah Allah will indeed forgive me and the sign for this is that you live a better life after repentance than you did before your repentance. This is the sign.
If you have been a sinner for a number of years, and then you repent, the future after that repentance should be a better future and that is a positive sign that Allah accepted the repentance.
Also, to feel always scared:
“What if Allah punishes me?”
“what if Allah holds me to task?”
This is a sign of faith.
If you ever forget being scared of Allah, if you ever feel safe, “Allah will forgive me no question about that”… this is a sign that you have gone very low, and this is a way to phrase it as follows:
Faith is inversely proportional; the actual level of iman is inversely proportional to the level that you think you have.
In other words, the more iman you actually have, the less iman you think you have; and the less iman you have, the more confident you have lot of iman. So the more iman you have, the more scared you are “Allah will not forgive me…” and that is a sign of Iman.
And the less iman you have, you say “Allah Ghafur, Rahman, He will forgive me” that’s not part of Iman.
So we’re always hopeful for Allah’s mercy and fearful of Allah’s punishment.
And inshallah that is a sign that Allah has accepted your repentance.