Do I tell my spouse when I have doubts about the relationship and whether I'm happy in it or keep my mouth shut and just accept sometimes I feel down and not trouble them with it?

No, I don't know in any way shape or form for sure whether I truly am happy in this relationship, there are ups and downs, but should I be telling my spouse when im concerned about the downs at the risk of making things even worse and the whole thing blowing up?

I do really like the idea of radical honesty in a relationship and just accepting that the truth is always right no matter if it hurts, but what if I say something stupid I just "thought" was true or dont explain right and make my life even worse?

What are you unhappy about? I think you should focus on these things yourself first and see their relation to yourself and feelings. Is it things about your partner or you? Maybe your partner feels this way too and you are both exhibiting feelings and mirroring each other. Perhaps it would be best to ask them genuinely how they feel about the relationship. What is strong and what is weak. If you care about this relationship you both can work through the troubles together. If not then you can always leave. Talk and listen to them before talking to the vast amount of retards here. I think if you ask about things and actually listen to what is said and focus on how you feel, you will find your answers.