Advice:
1. Set your bitrate (output panel) DOWN. You have 3000 -- too high. If your upload speed is 1800, then set it no higher than 1200 (you can raise it up later if needed, but first make it work). Also, lower your audio bitrate -- 128 should be fine, 256 is overkill.
1b. For twitch, raise your reconnect delay -- twitch recommends 10 seconds, and having it be too low will result in twitch refusing the re-connect, thinking you are still connected and broadcasting.
2. Set your encoder setting from medium to ultra fast. Lower it LATER, after you have verified that things work.
3. If you change output mode to advanced, you have a setting for keyframe rate. Twitch wants keyframes no worse than every two seconds, and sadly, the simple mode will not give you that guarantee (even with a 20 fps, and an advanced parameter of keyint=13, which should give three keyframes in 39 frames (two seconds), I still get twitch complaining occasionally that my keyframes are not coming fast enough).
4. Don't try to send at 1080p. Drop down to 720p, or 480p. This will cut bandwidth used by about 1/3rd each time you drop the pixel count down by 1/4th.
5. Downscaling: Again, start with the simple one (I think it's "linear"). Twitch recommends that, and it will save CPU.
If your quality isn't good enough, then you can adjust things.
First: Have a twitch window open on your computer, so you can look at the playback stats. Pay attention to the buffering, latency, and dropped frame rates. If these get bad (in particular, if the buffer keeps running out, or latency goes up, or if bitrate or FPS wander too much), you've pushed twitch past what it can handle from you, and you want to back things off.
Spare CPU? Better downscaling, or better encoder setting. Warning -- I had spare CPU, went to better encoder, and the result was stuttering, playback buffering, etc, even with leftover CPU and bitrate. Apparently, OBS's encoder can't go above 100% (one core) even if your system has lots of spare cores.
Spare bandwith? Better bitrate, or resolution, or FPS -- and back off the instant you start seeing dropped frames on the transmission.
