Dealing with a less assertive partner

I recently had my first trip with my SO where we spent several days together, and I'm realizing that they are much less assertive and reliable than I initially believed. This is a big problem for me, as I need to feel confident that the people close to me will be honest and forthright about any concerns they have with me. I can often be rude or hurtful without realizing, and I need to know that those around me will tell me if I do so. From the trip experience, it seems like my SO will encounter something that doesn't meet their expectations, and rather than address the issue, try to justify the problem on behalf of the other party or invalidate their own inconvenience. Clearly this issue stems from anxiety, but I can't let that behavior become acceptable in our relationship or there will be a lot of mistrust. How do I bring this up to them without making them feel defensive or inadequate?