Kaitlyn Bristowe is celebrating rumored boyfriend Zac Clark’s latest sobriety milestone. “15 Years Sober. Thank You For My Sobriety,” Clark, 42, wrote via Instagram on Sunday, August 30, alongside photos of the Bachelor Nation alum celebrating his achievement with a donut cake and candles. “God, grant me the serenity to accept the things I cannot change,” he continued, citing the famous “Serenity Prayer” penned by theologian Reinhold Niebuhr. “The courage to change the things I can and the wisdom to know the difference. Amen.” Bristowe, 41, commented on the post, “You’re my...