PM is a full-time job on its own, and it doesn't match up well at all with being a PO! You should probably try and find someone else to do one of those roles.
If your retros are super-negative, then try limiting everyone to one positive and one negative comment/post-it each per retro, and when you discuss the retro comments, talk through the negative ones first and the positive ones last so the retro ends on a positive note. Limiting people to a small number of comments forces people to consider the things that really got in the way rather than raising every little moan they have.
There are lots of different ways of doing retros, and as PM you are ultimately in charge of facilitating a useful retro. If the retro is overly negative then it becomes less useful.
If your retros are super-negative, then try limiting everyone to one positive and one negative comment/post-it each per retro, and when you discuss the retro comments, talk through the negative ones first and the positive ones last so the retro ends on a positive note. Limiting people to a small number of comments forces people to consider the things that really got in the way rather than raising every little moan they have.
There are lots of different ways of doing retros, and as PM you are ultimately in charge of facilitating a useful retro. If the retro is overly negative then it becomes less useful.