Don't beat yourself up, maybe you just don't like long haul trips? They are tougher than short haul, they feel more foreign, the weather is different, the time zones are different. However, you're there now!
I personally love all of that and embrace it, in Mexico we normally get up super early due to jet lag, and get in the pool before breakfast - it's peaceful and cool. We pick excursions that need early mornings, but then we go to bed fairly early and don't drink too much. I am never ever bored in Mexico, there's absolutely millions of things to do (ruins, jungle, nature, water sports, snorkeling, xel ha, xplor, xcaret, cenotes, boat trips) and we are out exploring every single day after an early pool morning. It's easy to book trips, or get taxis out and about to places.
I think you've picked some fairly tiring excursions there, chichen itza is a really long hot day (its amazing but if you're struggling with the heat, its a lot and i wouldnt recommend unless youre up for a long hot day). Maybe look at some easier going more local trips? We loved Xel Ha, it's basically an outdoor natural theme park with loads of snorkeling and water stuff so you stay wet and cool, it's easy and well organised and tourist friendly if you're feeling out of it. Or look at a local cenote trip, they're really fun and again, cool as they're underground. We also did snorkeling at Akumal, amazing to see wild turtles and you're in the sea so not hot, Tulum has beautiful ruins over the sea that are quick taxis to and then near a lovely beach to cool off and tulum town is nearby and has some bougie cafes. These are all very close to playa del Carmen so you'd be done in a half day. You can also get a taxi into town, explore the restaurants and bars, playa del Carmen isn't super exciting but if you're bored of the resort it's worth a try.
You need to make the best of it, but maybe long haul just isn't for you. Some long haul isn't a time difference, like cape town is the same times as the UK. Europe is also wonderful, sometimes it's the best place to be!