How can I figure out which part of my computer needs an upgrade?

Honestly, both your CPU and GPU could use an upgrade, especially considering the games you play. I’d recommend swapping out your motherboard for a new one that can support a better CPU. You'll likely need to upgrade the whole platform (motherboard, CPU, and maybe RAM) to really see a good performance boost. If budget is tight, consider picking up a used GPU later on, but do the other upgrades first.

At this point, it seems like you're in need of a complete replacement rather than just an upgrade. It's a bit of a bummer, but the cost-effectiveness just isn't there for upgrades with your current setup.

Your hardware is quite dated. The RAM configuration with four sticks may be causing some slowdowns, and your CPU is quite old by today's standards. The GTX 1050 Ti was entry-level when it launched, and it’s not holding up well with modern games. Unfortunately, just upgrading one component likely won't solve the bottleneck issue since everything is somewhat outdated.

If you're looking for something significantly better without spending too much, you might consider getting secondhand AM4 parts. You could find a Ryzen 5 5600X for around 40 euros and a compatible AM4 motherboard for a similar price. That could provide a substantial performance upgrade while still allowing you to keep your DDR4 RAM.

A good way to check for bottlenecks is to use the Steam Performance Overlay in Detailed Mode or download MSI Afterburner, which is free. Launch some games and observe the CPU and GPU usage. If your CPU is at 100% while the GPU is lower, then the CPU is holding you back. Conversely, if the GPU hits 100% and the CPU is lower, that’s where the upgrade should focus. Gathering this data is crucial before making decisions about upgrades, as sometimes the issue could be due to other factors like temperatures or RAM setup.