My father left us when I was small, yet he is still my wali. How can I accept this calmly?

Your hurt is understandable; accepting your father as wali does not mean you forgive everything or forget the wound. The position of wali is a ruling, not a reward for his deeds. Nor does it give your father the right to control your life after you marry. Think about what you need on that day. Perhaps you want him to appoint a marriage officiant so the meeting is shorter, or perhaps you want him to pronounce the offer (ijab) himself. Both are valid. Share your feelings with your husband-to-be so that he understands. If the old wound is too heavy, talking to a counsellor before the wedding day will help a great deal. See mental health.