I am still responsible for payments on a loan that I signed for a friend?

Yes.

If you signed for a loan, you are responsible to make payments on it.  If you fail to do so, you can get sued for it.  If you had a side agreement with your friend, that does NOT mitigate your responsibility, however, your friend may be liable to you for the amount paid by you.

Unfortunately yes, except in the very rare but theoretically possible case that the loan agreement, promissory note, etc. contained a deadline or limit in it--e.g. that you would only guarantee the loan for a certain amount of time, or up to a certain dollar amount. Otherwise, until the loan is discharged (whether paid off or otherwise, such as by bankruptcy), you will be responsible. You may have the option of making a claim or taking legal action against your friend, if (s)he doesn't pay and the creditor comes after you for the money--that depends upon the paperwork. For example, if the friend is also on the loan and doesn't pay, you could sue him/her; or if you are the only one on the loan itself, but the friend executed a separate agreement to you promissing to pay.

On the other hand, if you took out a loan for friend and he/she is not on the loan and did not otherwise promise to pay you, then the friend might not have any liability.