The Dean Court captain saw Lauri Dalla Valle volley Cherries in front but admitted Saints' 11th-minute leveller could have been avoided.
Pearce tried to shepherd the ball out of play but was dispossessed by Rickie Lambert, who crossed for Lee Barnard to fortuitously head home through Shwan Jalal's grasp.
Southampton went on to win the south coast promotion showdown 3-1 after second-half efforts from Dean Hammond and Lambert.
Discussing Saints' first goal, Pearce told the Echo: I thought the ball would go out, it held up and he (Lambert) nicked it, crossed the ball and they scored.
I have to learn from that and should have just kicked it out. It is disappointing but something you have got to learn from.
I have been delighted with my form this season but it was a little mistake. Mistakes happen and you have to learn from them and move on.
It did not cost us the game, but it cost us a goal. If that hadn't happened, you never know what might have happened. But it is all ifs and buts and you can't live life on ifs and buts.
Pearce added: I hope everyone at the club saw the effort the lads put in because I thought everyone gave 100 per cent and we were unlucky.
We will do our best and keep on going.
Boss Lee Bradbury added: Pearcey and Shwan have been different class for us all season so we can't really have too much of a moan about them.
They have both kept us in games on numerous occasions in the past.