Assalamu alaikum - struggling with tawbah and a heavy heart

Assalamu alaikum, brothers/sisters. I used to do things I stopped doing a year or two ago. They weren't major sins, but when I remember my past actions I feel overwhelmed with guilt, shame, and disgust. I've tried to draw closer to Allah - praying more, especially Tahajjud - yet my heart still feels heavy and hurt, full of guilt. I read reminders about Allah's mercy from the Qur'an and Sunnah and sometimes they help briefly, but the comfort doesn't last. One thought keeps coming back every time I make istighfar and ask Allah for forgiveness: if someone performs sincere tawbah properly - feels real regret, resolves never to return to the sin, and continually seeks forgiveness - could they still be sent to Jahannam afterward just to “pay the price” for what they did? That fear terrifies me. I honestly don't understand what led me to do those things. Thankfully it was between me and Allah and I didn't harm others, so I don't know why this burden stays so heavy in my chest even though I'm taking the right steps to fix myself. Sometimes an inner voice tells me it's all pointless and I'll be doomed anyway. Please make dua for me. If anyone has advice or Qur'anic/sunnah reminders about repentance, hope, and Allah's mercy that helped you feel at peace, I'd appreciate hearing them. Jazakum Allah khair.