I believe if you answer a few questions we can be more helpful.
1-Do you know anything about music?
2-Do you know what chords you are writing?
3-Do you know how to make a scale?
4-Do you have any music creating software?

If you don't have Band-in-a-Box you might consider it. You would write your songs like you do now, bring the MIDI file into Band-in-a-Box, select a style and have it generate a bass line.

Or write a song sans the bass line, post it here, and have people write a MIDI bass line for you to use and/or study.

Please give us a good description of what you know about music and we can be more helpful.

PS - don't feel put down if you don't know music. I had a friend whom knew nothing about music. He didn't know chords, scales, or any theory what-so-ever BUT he produced some of the most beautiful music I have ever heard. He recorded his keyboard via his computer and did track after track. He did everything by ear.

I'm no longer allowed to go caroling at the psych hospital.
I guess Do You Hear What I Hear was a bad song choice!

64 bit Win 10 Pro, the latest BiaB/RB, Roland Octa-Capture audio interface, a ton of software/hardware