Welcome to the forum Lucas!
"How can I come up with good ideas?"
That's the age old problem for artists of every type of art; from poets to tattoo artists. Every composer has their own way of approaching this. You might be inspired by a chord progression, or a fragment of a melody, or hear a beautiful hymn and want to create your own. I think when you are confronted by an empty, lifeless manuscript paper, or a newly-created, empty midi file it can be a bit daunting. No different than a poet sat by candlelight with his quil and ink pot faced with the infinite expanse of his plain empty paper.
But, there is music in you! You said your favourite composers were Bach and Vivaldi, they too would have had this problem from time to time. How did they come up with good ideas? An empty stomach sure is a good motivator for an aspiring young composer, they didn't have much choice but to face the frustration and dig deep.
You may have 10 ideas but only one of them is any good. It's a bit like mining for diamonds on Minecraft, you'll dig through a lot of rock before you find anything of value and that's OK.
When Haydn heard Mozarts six string quartets he remarked that Mozart had such good taste. He didn't say how clever his music is, or how beautiful his harmony, he referred to his taste, meaning his ability to discern good ideas from bad ideas. That's interesting. The question could be reframed, "How do I discern good ideas from bad ideas?"
That takes 10,000 hours of experience, trial and error, perhaps talent too. There are so many unfinished fragments from the great composers, you won't get to listen to them on the radio, but if you did you'd realise they had this issue too. Although to be fair, their castoffs are of such good quality, but they were chasing a higher art form, that us, as beginners needn't concern ourselves with.
To point you in the right direction reflect on the fact that a motif is the unit of thought in music. We can all hum or sing a short musical idea, try it. The motif from Beethoven's fifth da da da daa. From that idea sprung a symphony. If you try to invent an entire melody spontaneously it most likely wouldn't make much musical sense.
So come up with many short one or two bar ideas, let's say 5 of them, then with you watchmakers eye, scrutinise them and cast off fools gold and keep the one that you believe is half decent at least. Then develop the idea, maybe come up with 5 more ideas that develop it to 4 bars. Delete 4 again, keep 1.
Another way to tackle this problem is to have goals and constraints. Choose a form, let's say a minuet, do some research to see how long they typically are, do they typically modulate at the end of the first 8 bars? What chord do composers typically use at the beginning of the B section? How long are B sections? How much material from the A section do they use in the B section? Once you have an inderstanding of a typical baroque minuet that will create constraints which will narrow the infinite expanse of possibilities.
Then, what mood do you want to create? Some minuets are quite sad, some are pompous, some are naive and playful. Again, that will create a limiting constraint. Then, what key would suit the style of minuet? Grave but dignified might suit G minor for example.
Little by little, constraint by constraint, you will build a discerning ear that will cut down any roots of mysical ideas that clearly do not fit the mould.
In the beginning, being a composer is more akin to learning to juggle than a brooding artist trying to put his soul on paper for the world to admire. Its a craft no different than that of a bricklayer building a house or a blacksmith fashioning a horse shoe. Art comes later once you're a master of the craft.
Apprentice yourself to others on here that are a little further down the path. We are all really helpful, we were all absolute beginners at one point.
If you would like me to take a more critical eye to your scores I can help you learn a few rules that will help guide you and point out a few errors? I found that helpful when I first started uploading scores on here.
Darren