How can I stop feeling guilty that I’ve taken the lives of so many insects in my life unintentionally

Firstly, if you unintentionally killed anything, there is no consequence of kamma.

"Intention, I tell you, is kamma. Intending, one does kamma by way of body, speech, & intellect.
Nibbedhika Sutta

The story of blind elder monk Cakkhupala from Dhammapada 1 shows the importance of intention:

On one occasion, Thera Cakkhupala came to pay homage to the Buddha at the Jetavana monastery. One night, while pacing up and down in meditation, the thera accidentally stepped on some insects. In the morning, some bhikkhus visiting the thera found the dead insects. They thought ill of the thera and reported the matter to the Buddha. The Buddha asked them whether they had seen the thera killing the insects. When they answered in the negative, the Buddha said, "Just as you had not seen him killing, so also he had not seen those living insects. Besides, as the thera had already attained arahatship he could have no intention of killing and so was quite innocent."

Verse 1: All mental phenomena have mind as their forerunner; they have mind as their chief; they are mind-made. If one speaks or acts with an evil mind, 'dukkha' follows him just as the wheel follows the hoofprint of the ox that draws the cart.

In the Abhidhamma, regret and remorse over the past is considered an unwholesome mental factor.

Worry (kukkucca) is remorse, brooding, and repenting over evil acts done in the past or good acts left undone.

The following theme for reflection from AN 5.57 (quoted below) can prevent the arising of unarisen unwholesome states. It encourages shame of evil (hiri) and fear of evil (otappa), which are mentioned as beautiful mental factors in the Abhidhamma.

So, remorse and regret over the past is unwholesome. But shame of evil and fear of evil, or basically moral shame (hiri) and fear of wrongdoing (otappa), for the future, is wholesome.

“And for the sake of what benefit should a woman or a man, a householder or one gone forth, often reflect thus: ‘I am the owner of my kamma, the heir of my kamma; I have kamma as my origin, kamma as my relative, kamma as my resort; I will be the heir of whatever kamma, good or bad, that I do’? People engage in misconduct by body, speech, and mind. But when one often reflects upon this theme, such misconduct is either completely abandoned or diminished. It is for the sake of this benefit that a woman or a man, a householder or one gone forth, should often reflect thus: ‘I am the owner of my kamma, the heir of my kamma; I have kamma as my origin, kamma as my relative, kamma as my resort; I will be the heir of whatever kamma, good or bad, that I do.’
AN 5.57

Hiri and otappa are explained below in Ven. Bodhi's "The Guardians of the World" :

The Buddha points to two mental qualities as the underlying safeguards of morality, thus as the protectors of both the individual and society as a whole. These two qualities are called in Pali hiri and ottappa. Hiri is an innate sense of shame over moral transgression; ottappa is moral dread, fear of the results of wrongdoing. The Buddha calls these two states the bright guardians of the world (sukka lokapala). He gives them this designation because as long as these two states prevail in people's hearts the moral standards of the world remain intact, while when their influence wanes the human world falls into unabashed promiscuity and violence, becoming almost indistinguishable from the animal realm (Itiv. 42).

While moral shame and fear of wrongdoing are united in the common task of protecting the mind from moral defilement, they differ in their individual characteristics and modes of operation. Hiri, the sense of shame, has an internal reference; it is rooted in self-respect and induces us to shrink from wrongdoing out of a feeling of personal honor. Ottappa, fear of wrongdoing, has an external orientation. It is the voice of conscience that warns us of the dire consequences of moral transgression: blame and punishment by others, the painful kammic results of evil deeds, the impediment to our desire for liberation from suffering. Acariya Buddhaghosa illustrates the difference between the two with the simile of an iron rod smeared with excrement at one end and heated to a glow at the other end: hiri is like one's disgust at grabbing the rod in the place where it is smeared with excrement, ottappa is like one's fear of grabbing it in the place where it is red hot.

And from Iti 42:

"Bhikkhus, these two bright principles protect the world. What are the two? Shame and fear of wrongdoing. If, bhikkhus, these two bright principles did not protect the world, there would not be discerned respect for mother or maternal aunt or maternal uncle's wife or a teacher's wife or the wives of other honored persons, and the world would have fallen into promiscuity, as with goats, sheep, chickens, pigs, dogs, and jackals. But as these two bright principles protect the world, there is discerned respect for mother... and the wives of other honored persons."

Those in whom shame and fear of wrong
Are not consistently found
Have deviated from the bright root
And are led back to birth and death.

But those in whom shame and fear of wrong
Are consistently ever present,
Peaceful, mature in the holy life,
They put an end to renewal of being.
Iti 42

Also, see Four Right Exertions.

Your guilt doesn’t do you any good and is just another ego trip. Your guilt isn’t helping any of those insects either. By just breathing you are killing countless sentient beings. That is the nature of samsara.

Instead of feeling guilty - which is a negative emotion involved with ego - you could instead feel regret for any intentional killing you’ve done in the past and resolve to no longer kill with intent or malice. That would be healthy and not involve a big ego trip.

Let go of your useless guilt and tell your ego to give it a rest and simply do your best to not intentionally harm sentient beings. Develop compassion for yourself and then stop the ego trip which is just causing you to suffer while not helping anyone or anything.

I’d say try and develop a sense of wisdom that will laugh at your silly ego and the oh-so-heavy shenanigans it is playing with useless guilt.

BTW, I was once on that exact same ego trip and when I told my teacher he gave me a big hearty belly laugh full of compassion and basically told me what I told you above :)