How do I let go of guilt from my life before Islam?

Yes, you can let go of it, because Allah has already wiped your past clean. Islam erases the sins that came before it, so you don't have to keep carrying them. Guilt that pushes you toward good is healthy. Guilt that keeps you stuck in the past isn't something Allah asks of you.

Islam wiped the slate clean

When the companion 'Amr ibn al-'As came to accept Islam, he hesitated and asked for a condition: that his past be forgiven. The Prophet ﷺ answered: "Are you not aware of the fact that Islam wipes out all the previous (misdeeds)?" (Sahih Muslim 121).

'Amr had fought against the Prophet ﷺ himself. If his past could be wiped away, so can yours. See Are all my sins forgiven when I become Muslim?

Allah can even turn it into good

The Quran goes further than forgiveness:

إِلَّا مَن تَابَ وَءَامَنَ وَعَمِلَ عَمَلًا صَـٰلِحًا فَأُو۟لَـٰٓئِكَ يُبَدِّلُ ٱللَّهُ سَيِّـَٔاتِهِمْ حَسَنَـٰتٍ "Except for those who repent, believe and do righteous work. For them Allah will replace their evil deeds with good." Quran 25:70

Your past is not a stain Allah keeps looking at. Your turning back to Him is what He sees.

Healthy regret vs. heavy guilt

Feeling some regret is natural, and it can keep you humble and grateful. But there's a difference:

Healthy regret says: "I'm glad I left that behind, and I want to do better."
Heavy guilt says: "I'm dirty, I don't deserve Allah, nothing I do counts."

The second voice isn't humility. Despairing of Allah's mercy is something the Quran warns against (Quran 39:53). If shame is crushing you, that's a sign to be gentler with yourself, not harsher.

What about people I hurt?

Sins between you and Allah are forgiven. If you wronged a person, for example by taking money, scholars say you should try to make it right where you reasonably can, without putting yourself in danger. Often a sincere apology or quietly returning something is enough. If you can't find the person, give charity on their behalf and make dua for them.

You don't have to confess old sins publicly or to anyone at all. Allah covered them, so you can keep them covered. See Do I have to tell my future spouse about my past?

Practical steps to let go

Make one sincere tawbah for the past, then stop re-confessing it every night.
Replace the memory with a deed: when a past image comes up, say "Astaghfirullah" once and do something good.
Remember the Prophet's ﷺ words: Allah would replace a sinless people with "people who would commit sin and seek forgiveness from Allah, and He would have pardoned them" (Sahih Muslim 2749). Turning back to Him is what He wants from you.
Talk to a therapist if guilt is tied to trauma, addiction or intrusive memories. That's not weak faith; it's using the help Allah made available.

Key points

Islam wipes out all the sins before it.
Allah can turn past wrongs into good deeds.
Repair harm to people where you reasonably can.
Constant shame isn't required, and it isn't humility.

Tip: Write down the one thing from your past that weighs on you most, make dua about it once, then tear the paper up. Let that be your reminder that Allah has already closed that page.