How did you create your Segwit address? I assume that you're using a P2SH-P2PWKH address (starts with 3). If so, you should have a private key that corresponds to that address as those P2SH addresses are generated from your WIF private key.

You have to download Electrum, and follow this guide[1].



[1] https://bitcointalk.org/index.php?topic=3383265.0