import re
s = "1. First sentence. \n2. Second sentence. \n1. Another sentence. \n3. Third sentence."
l = re.findall(r'(?:^|\n)(([0-9]+)\.[\s\S]*?)(?=\n[0-9]+\.|\Z)', s)
curr_num = 0
result = []
for s,num in l:
	if curr_num > int(num):
		if not result:
			result = ['']
		result[-1] += s
	else:
		result.append(s) 
	curr_num = int(num)
    
print(result)