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UTG+2 is a tight and aggressive player and certainly knows what he is doing.

Cryptologic 0.50/1.00 Hold'em <font color="#0000FF">(10 handed)</font> link

Preflop: Hero is UTG with Q[img]/images/graemlins/heart.gif[/img], Q[img]/images/graemlins/diamond.gif[/img].
<font color="#CC3333">Hero raises</font>, <font color="#666666">1 folds</font>, <font color="#CC3333">UTG+2 3-bets</font>, MP1 calls, <font color="#666666">1 folds</font>, MP3 calls, CO calls, <font color="#666666">3 folds</font>, <font color="#CC3333">Hero caps</font>, UTG+2 calls, MP1 calls, MP3 calls, CO calls.

Flop: (21.50 SB) 7[img]/images/graemlins/spade.gif[/img], 3[img]/images/graemlins/spade.gif[/img], A[img]/images/graemlins/heart.gif[/img] <font color="#0000FF">(5 players)</font>
<font color="#CC3333">Hero bets</font>, <font color="#CC3333">UTG+2 raises</font>, MP1 calls, MP3 calls, CO folds, Hero calls.

Turn: (14.75 BB) 4[img]/images/graemlins/heart.gif[/img] <font color="#0000FF">(4 players)</font>
Hero...

What do I do? There are 5 players in this huge pot! Somebody must have an ace.....?

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Unfortunately, there is nothing you can do here. You're right, someone must have an ace, especially given that two poeple called two bets cold on the flop. On the turn you should probably check/fold, although if UTG + 2 bets and gets two calls, you may have implied odds to call and try to hit your set. Overall I would lead towards folding though.

I'm pretty sure you shouldn't lead out on the flop, but I can understand why you would do it in the middle of the hand.

I think the most important thing to do after hands like these is to remain confident that you did the right thing by capping pre-flop, and not let the hand get you down.