There is no way to 'merge' it. On some newer devices, ones running Android 6.0 or newer, you can make the SD card "Internal" storage, however it is not without its drawbacks.
On that device your best option would be to move things like photos, videos, and other files over to the SD card, or better yet off the phone totally an onto either cloud storage or a computer. Safer that way. This will free up quite a bit of space on the phone.
You may also want to consider clearing out the things you don't really need. People ted to keep all texts, browser history/cache and other such things that they don't actually need. If you they aren't things you really need, I would suggest clearing those things out too, which will free up more space. Then uninstall any apps you don't really use (and that are not system required), restart (turn off and back on) the phone and you should have quite a bit more space.
In the future save things like photos, files, etc., to the SD card instead of the phone.