Will Allah accept my fasting and prayers after I looked into horoscopes, even though I repented and said that I do not believe in anyone but Him? And will my prayers be invalidated for forty days?

What you did is reprehensible. A Muslim must avoid looking into the unseen and believe that none knows it but Allah. If you repent and regret, we hope that there is nothing against you, for the one who repents from a sin is like one who has no sin. Repentance is accepted immediately, not after forty. The hadith that mentions the prayer of a wine-drinker not being accepted for forty mornings is interpreted by scholars as a lack of divine aid (tawfiq), not as a non-acceptance of repentance itself. Repentance is accepted for all sins, even major shirk.