Kaizen. Small improvements. Start now and don't deviate.
You grow by constantly pushing yourself just barely outside of your comfort zone. No anxiety or panic needed, just a little tiny dose of discomfort.
Start now, with the smallest improvement you could possibly make. Then continue to make one small improvement every day.
Are you a complete shut in? Make it your goal to spend just 1 minute outside in public. If 1 minute is too much, then make it 10 seconds. Do it again tomorrow, and add just a little more time. Did you feel a bit uncomfortable? Good. If not, push yourself just a little more.
Are you good with being outside? Make eye contact with one person for 1 second. Tomorrow, make eye contact with two people. Or try to hold eye contact for 2 seconds.
Good with eye contact? Let's get talking.
Go to the grocery store and use the human checkout line instead of the self-checkout. Then the next day, make it your goal to say one sentence to the checkout person beyond the usual pleasantries. A comment about the weather will be just fine. Once you're good with talking about the weather, ask the checker if they have big exciting plans for this evening/the weekend/Christmas/whatever.
Just take action every day. ACTION. EVERY. DAY.
If you feel like you're not making process fast enough, or if you slip up one day and stay firmly within your comfort zone, don't beat yourself up about it. Just get out there tomorrow and make one small improvement. Small improvements will have a huge effect on your quality of life before you even realize it.
I've been there. It's possible. Start now and don't deviate.