“I've come a long way since then.”

You are a web designer. You are looking at a website that you designed 10 years ago. Now it looks really old and unattractive. You say:

I've come a long way since then.

come a long way

To "come a long way" means to grow or show improvement:

You're still not quite ready to compete, but you've come a long way.

since (some event)

"Since" is used to talk about something that has happened after a certain time in the past.

You use "since" with "have":

I've loved him since I was 10 years old.

Haven't you eaten anything since lunch?

You don't use "since" with simple past tense verbs. Remember this! For example, "I loved him since I was 10" isn't correct.