I often travel with my kids (school age) and have since the older one was a baby. Kids are a recipe for a trip not going as planned, so I've learned a little bit about what works for me.
-Make lists of three tiers - must dos, want to dos, and if-possibles. Have, at most, one must do per day with some days of no "musts". Limit the musts to true essentials for you - if you don't care all that much about the eiffel tower, it's not a must even if everyone else thinks it is. Be extremely selective about the "musts", and be happy if you get to a want-to-do instead of mourning all the if-possible's you missed. The idea is that you need to be OK only seeing the musts and missing everything else (although you will have time for some of the wants as well, especially if you take the next point on my list!).
-Take more time per place - less moving around means more time to make up for "mistakes", and you feel more comfortable in your spot. Also, pad your time. A lot. If your train arrives at 3 and it's a half hour to the hotel...plan on it taking an hour or more. Less stress about "delays" and you won't schedule something too close. Get there early? Great, you have time to relax or to go to the want-to-do down the street. Bonus!
-try your best to remember that the things that go "wrong" make good stories and memories later. No one cares about my trip to the Louvre - it's been done by plenty of people - but they love my story of getting caught in a rainstorm, falling and twisting my ankle (mildly), accidentally limping into a hospital that I thought was a hotel, and getting driven to my hotel by the doctor, who didn't speak English, and my French was limited to 20 words. If everything turned out OK from the hospital, this may be your story!
-I don't really know how to do this, but I've found that over time focusing on experiences not "sights" to be extremely helpful. Just wandering around Venice with no plans to go into a museum or eat a spectacular meal. Stop at a cafe to enjoy the scene, but don't worry about the meal living up to expectations. You're in Venice - savor it, don't run to get to the 10th church of the day before closing. More of an in-the-moment kind of approach. For ME, the only way to do this is a combo of careful planning (so I know the must-sees and can feel comfortable with skipping other things) and travel experience. On your first trip, it's hard to have this perspective, but if you keep it in mind it will develop. I'm still working on it, and some days I'm terrible at it, but it's coming.
-Re-live your trip in your mind after you get back. Right away, and months later. See what stands out to you. Then do more of that kind of thing, less of the things that don't stay with you. This will vary by person, but whatever sticks with you is what matters for your travels. On a trip recently, one of the things that stands out most is a day in a park with a beautiful view, kids playing with local kids on a playground, going to a neighborhood store to get a picnic and just being there. So, more of that.