That’s a great example of WHY you need to vote in every election: it’s not enough to get a narrow win, you need a margin to get much done. If you want candidates more to your liking, you need to start organizing at the local level and building a movement: politics go to people who show up. The GOP has minority approval for most of their positions but is in power because their voters show up at every election.
In the United States, one especially important thing to focus on is electoral reform. Currently, most voters have a binary choice where not voting for the Republican candidate is effectively the same as voting for the Democrat or vice versa. The best long term thing we could see would be more ranked or run-off voting so it’s viable to run multiple candidates.
How could they? They didn't have the votes, because people didn't vote for democrats. You want democrats to have power in the government: make sure they have a majority.
I'll also note that your vote for president has no impact on the senate's membership. It's entirely a non-sequitor.