Why commercial partnerships fail so often

Dear everyone, I'm new on the forum so I'm not sure it's the right thread to post, otherwise, please apologize :)

I recently separated from my former employer and decided to start my own business. I was quickly contacted by a supplier that I often contracted when I was working in my old company. She had a similar idea, she only had accounts that wanted to expand the business to areas in which she had no experience and which were to be my specialty. We talked about opening a business that would cover this business area. I have the know-how and she has the buyers. We have already started working on some things, but nothing official at the moment. In addition, I must mention that our commercial relations in recent years have always been good and we have even become friends. During our meetings, it became clear that we seem to share vision and enthusiasm.

However, very often, this person is difficult to reach (by phone, mail, etc.), it seems that she has many other responsibilities and / or has difficulty prioritizing her. This has intensified in the last month and I am beginning to question his dedication to the project.

On the other hand, today I had a conversation with an old friend and colleague who had been pushing me for years to start a business. Even today, she insisted that we do something together.

My biggest concern is "What does it cost me?". The person I first described has business and contacts, and she is the kind of person / professional who excels in everything she does, very capable but easily distracted. The other person has no buyers or contacts, but is very consistent and has good management skills.

Is it wiser to go with the person who has contacts and business ready (knowing that I will have to invest extra energy in order to keep it on track) or the person who does not have the energy? brings nothing in the transaction but who is a strong and focused person? I should add that I trust them too.

Or, would you advise me to start alone?

I would be very grateful for your thoughts on this. Thank you in advance! :)