Here's a question for you - what sort of structure are you providing to encourage your people to perform? Do you have a mutually beneficial and understood verbal or written contract that specifies who is responsible for what, what kind of work each person is responsible to put in to be counted as a full participant, and what kind of rewards they can expect for staying the course?
People are people, regardless of how smart they are. They lose interest quickly, get distracted, and often after a while begin to look for reasons to put in less than their best efforts, especially when the payout for their work is seen as iffy or a long way off. Very few are disciplined enough to work consistently for long periods of time towards a goal without some incentive along the way to continue to do so. Could this be what needs to be tweaked, or maybe has been missing altogether, in your last three efforts?
So, to answer your two questions, here is what I would do.
If you want to proceed with your start up, make sure that you have a clear understanding of the roles and work required to pull it off, and put this down somewhere in writing. Perhaps in your business plan. Then be sure that anyone who signs on from now on understands what is expected of them and what the rewards will be and the consequences for not following through. Interview potential partners like you would a potential employee to make sure they have the background and skills required to deliver. Offer incentives for good work. Get something in writing for your protection and for the protection of your new partners. A memorandum of understanding may work just fine, or a simple contract. Be sure to invest a bit of cash in having it vetted by a business lawyer before you sign it or ask someone else to sign it.
If you want to proceed without partners, then I'd start looking for some sharp folks who are willing to freelance for you for wages. Then I'd take my completed business plan and start shopping around for capital... investors, loans, savings. There are whole books on how to do that, and many are industry specific.