Hi,
As per Sagaleyar, no remedies (Prayachitham) are available for Sins committed knowingly. However, Manu has modified that Remedies can be performed even for sins that has been committed knowingly (only for certain sins).
I understand that there is a provision during Avani Avittam (Kamo karsheeth japa) to seek pardon for not doing Sandhya vandhanam on exceptional basis. In your case, it has been not done regularly and you are aware of it.
It is better to start and continue this on regular basis with dedication & determination. All the best!