Revised manuscript take too long to process
First things first, relax. You are worried about the status of your paper, but nothing out of the ordinary has happened.

When you submitted it, the editors of the journal sent it for review. From your description, there were at least two reviewers. Peer reviewers are volunteers, so editors cannot force them to submit reviews. They can invite them of course, but whether or not they actually submit a review on time isn't something that can be directly controlled. From your description, one of the reviewers hasn't submitted a review. This is perfectly normal. It's not desirable, but it's normal.

What happens next is that the editor can either look at the review(s) that were submitted and make a decision, or wait. Again from your description, the editor has chosen to wait. How long will the editor wait? Only she will know, but you can be confident that she won't wait longer than ~1 month. If the reviewer still hasn't submitted a review more than one month after the deadline, the odds are he will never submit one. Maybe he forgot about it, maybe he was too busy, maybe he had some serious personal issues that stopped him from reviewing, who knows? It happens often enough that editors have probably dealt with it before.

Here's what will likely happen to your paper.


The reviewer submits a review.


Everyone is happy, the editor makes a decision, and you move on.

The reviewer doesn't submit a review.


The editor could make a decision based on the reviews she's received so far. This depends on how good those reviews are. If they're convincing enough then she might decide.
If they're not convincing enough then the editor could decide to invite new reviewers. This is the nightmare scenario for you, since it'll easily add another 1+ month to the review time. There's nothing you can do about this however except for withdraw your paper to submit elsewhere (in which case you'll have to put up with another 1+ month of review time anyway).



One thing that definitely won't happen is that your paper will be reviewed "forever". Eventually the editor will stop waiting and make a decision, especially if you remind her. As for the other thing that worries you, if your paper is indeed rejected, it's not likely to take a year. So far it's been two months, which in the physical sciences is very much within the norm. Chances are you'll receive a decision in another month.

As for what to do: do nothing and wait. You cannot rush the reviewers (you don't know who they are anyway) and you cannot rush the editors (it's their journal, things are only ever published when the editors are satisfied). If it goes another month+ without word from the editors, then you can send them a reminder, but otherwise just keep waiting.

If you are really concerned, don't take my word for it. Speak to your supervisor. Ask him how long it takes in his experience to publish in your field, or how long is a reasonable time to wait before worrying. If your supervisor isn't worried, you shouldn't be worried either.