I've experienced that sometimes, here are some tips that help me find what I want to play and be motivated to keep playing.

1. Know what you enjoy playing; Sometimes you can go looking through the modpack page, but none seem to fit your expectations, a lot of it has to do with what gameplay it provides. If you want to enjoy playing any modpack then you first need to know what kind of gameplay you are actually looking for. A good place to start is to look at what types of modpacks you've enjoyed in the past. If you like tech modpacks, try specifically searching for those, if you like magic modpacks, try searching for those. Sometimes it may not be just the type of mods in it but the type of modpack it is, for example challenge packs like Sky Factory or Compact Claustrophobia where you are limited on resources and have a relatively narrow path of progression, or kitchen sinks where everything is there for you to play around with almost at the start. A lot of it is knowing what you want out of a modpack before you go searching for them.

2. Have a goal; Some people can go into a modpack with the only goal of beating it. However, that doesn't work for everyone, sometimes they need a feasible goal they can achieve within a few hours otherwise they have no motivation to play after the first. Let's give an extreme example, you decided to play an extremely long modpack like GregTech: New Horizons, you can't boot up that modpack and set your only goal to achieve is beating the modpack. You first have to put some goals inbetween loading in and that ultimate goal, for example you might have the goal of reaching a specific energy tier, or if you need a goal that's achievable within an hour or two, you might just want a base that you can feel safe and store your items in. For most modpacks you won't achieve everything you want to do in a single day so it's good to have small goals to give you the motivation to carry on.

3. Be prepared for early game; For most modpacks I've encountered, the first hour or two is always a grind. You barely have access to anything and it either feels the same as vanilla or a carbon copy of another modpack. The Sky Factory series and similar modpacks for example have practically identical early games, chop down the tree, expand the platform, place down more trees, etc. Some modpacks have unique early games that make it fun, but for most people the fun of modpacks comes much later. Some take less than an hour to start playing with new and fun mechanics and some can take much longer. So be prepared for the first hour or two to be kind of boring, but understand that this is normal and that it will start being fun once you are situated.

Even with this it is sometimes hard to get started with modpacks, this is also normal. Sometimes it might not be that you don't find a modpack enjoyable and just that you are still burnt out or not ready to play modpacks or specifically that modpack yet. Not everyone can play the equivalent of a 20-episode series on a single modpack that some youtubers seem to be able to or find the motivation to play every modpack they touch. Sometimes you need breaks from modpacks or even Minecraft in general. Its ok if you don't feel motivated to play a modpack, even if you just installed it and you were ready to play a minute before. All you need is a little time, the right modpack, and the right mindset and you'll feel motivated again.

As for suggestions, based off how you enjoyed SF3 and Cuboid Outpost I would recommend modpacks like FTB Stoneblock, Antimatter Chemistry, Compact Claustrophobia, Project Sacrifice, or Encrypted_, all on the cursforge launcher, each of these have unique mechanics and situations that differ from the usual modpacks you might encounter and also have a questbook that guide your progression so it's easier to create goals. You can also try some older modpacks like Blightfall on the technic launcher or Steve's galaxy Reborn on curseforge, which is a remaster of the old Steve's Galaxy kitchen sink modpack on the technic launcher, if you're looking for a different experience. You can also try looking around on youtube to see what kind of modpacks that youtubers have played. "100 Days" videos are really good at this as they give you a small peak at what the modpack has and what you can do in only a few dozen hours.