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.

Whenever I get something stuck in the back of my throat, I dislodge it by drinking a beer.
It's called the Heineken Maneuver.

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