I think you are experiencing difficulty with "letting go".
Particularly with DMT and LSD it is crucial to the experience that you fully let go of everything and accept what you see and what you get, for what it is. There is a literal loss of power during these experiences. You are no longer in charge but are being guided by the substances. Some believe there is a spirit behind these experiences, personally I think it's all just chemical. Either way if you fight the experience you will lose. You cannot be in charge when looking into the pool of infinite questions. You are a passenger at that stage, not the driver.
You literally have to accept that your reality is your perception right in the moment of the journey, and let go of your family and everything that you have created. That is not you. You are your perception during these types of experiences, not your "life". Some may contend that that is true at all times.
You will always come back, even if you take a devastating dose. You may be changed, but you always come back. You already know this. But fear is preventing you from letting go.
I also have children, a wife, home, career... all the things that we build our lives around, so I'm speaking from a point of reference where I can understand what you fear. But that is all it is. Fear.
So the thoughts you loop on, let me try and answer them logically for you.
1. Am I real? - Do I exist or am I just a figment of my imagination.
A: It doesn't matter. You wouldn't be able to tell anyway. Accept it.
2. If I do exist do any of you exist as well or are you just a figment of my imagination.
A: It doesn't matter. You wouldn't be able to tell anyway. Accept it.
3. Did I just create all of you (imagination wise) so that I wouldn't be lonely?
A: It doesn't matter. You wouldn't be able to tell anyway. Accept it.
4. Am I the only one who exists - being as we are one universe and we are all one - am I the only mass galactic creature out there.
A: You are at the center of your universe, and you are not in control. I am at the center of my universe and experience the same. We all experience the same, but you are alone. You were born alone and you will die alone. Everything here is temporary.
5. If I do let go and go traveling - will I always definitely get back?
A: Yes, always. You may not be the same when you come back. You may return with more questions or answers than you had when you left, which may cause you to change how you perceive/approach life... but you will always come back.
I hope you can learn to let go. You may have become too attached on what is "real" to openly accept the possibilities of other realities that can be generated by these experiences. Fear is always the enemy with psychedelics, acceptance is always the path to enlightenment.
You are, you were, you always will be. Everything else is temporary.
The nearest we ever come to knowing truth is when we are witness to paradox.