I can't give my boyfriend children

You might still be able to adopt (although it would likely take longer). Plus your boyfriend can (and I hope already does) have a meaningful relationship with your children. Let's also step back a bit and recognize that your younger child will be 17 years older than any child you conceive today (assuming your tubal ligation surgery was magically reversed in the next 2 minutes). That is a VERY long time between kids. Your elder children probably will have more of a relationship with each other than with any as-yet unborn children, given the near-generational difference.

Beyond that, I hope you have other aspects to your relationship beyond children.

Talk to your boyfriend. Find out where his head is - and while he's sober.

Ha! In 5 years, you might be a grandma.

then you probably can't afford to put a third child through college/university either, so stop while you're ahead

have a serious talk with your boyfriend about all of this

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I was your age about 20 years ago and chose to end a relationship with a younger partner who I felt would regret not having children. By that time I had made the decision not to have biological children. He always said he was fine with it. Now I wonder if we should have talked about the adoption route more - I was the one who kiboshed the idea since I was trying to push him toward having his own biological children.