I'm sorry it's been so hard for you.
Where is it hurting? Is it inside or outside? My first couple of times hurt awfully bad, more so than most of my friends, and I bled a lot. I seriously wondered why people want to do this more than once.
Are you relaxed? Is your boyfriend making sure you are aroused? Is there foreplay?
What was it about the pool? Was it the position? The atmosphere? Was it spur of the moment, so you didn't have a lot of time to think about it?
If he's having a hard time entering you, has he tried using fingers first? Is he going slowly? Not to be crass, but just ramming it in isn't going to work. Lube is awesome - keep using it. Make sure it's actual lube and not lotion or something, and if you are using condoms, make sure it's water based, so it doesn't destroy the condom.
This will sound stupid, I know, but try to relax. If your muscles are tense, it's going to make it that much harder. If you are old enough, have a glass of wine first (just one or two - you don't want to be drunk for this, and alcohol is dehydrating, and can dry you out - the opposite of what you want here), light some candles, take a bath together - whatever relaxes you.
If it still hurts after your first few times, check with your doctor to make sure everything is okay.
Most importantly, make sure you are emotionally ready for this, with this guy. If you aren't, your body may be responding accordingly.
Have you ever had a pelvic exam from a doctor (the kind that uses a speculum)? I'm curious to rule out some kind of physical issue such as a very tough hymen.