So I'd say yes, do it, but simultaneously be on the lookout for a potential co-founder. If one arises, great. If not, don't let it be an excuse not to follow you dream.
Partnerships consisting of staying on the same page for multiple years, in one's 20s, is harder than marriage. People, priorities and capabilities change too much.
On the other hand having someone join a vision already in progress can help with keeping alignment.
No one got anywhere by doing nothing.
I would think its better to try and fail.
1) If I carry on with the product, can I keep quality and speed to improve sales significantly or will I quickly hit a wall which will cause my customers to leave?
2) If I find a good founder instead, will they help me with stuff I am not good at and give me the space to keep quality and speed or is it a distraction?
There's no one-size-fits-all solution for founders. Co-founders come with their own set of issues--whether you're on the same page about the company's goals, whether your skill sets are complementary or not, what kind of long term commitment you both have to the problem space, etc.