Your veins change, long-term, with increasingly long term IV use. You seem to know the basics of IV hygiene (#1, using a new needle every time.) A fairly minor point that not everyone is aware of is that not swabbing with alcohol after injecting and letting some time pass after swabbing initially (or using a sterile cotton/gauze) can actually help, and swabbing after use does not really make much of a difference; better to put a sterile gauze/cotton on and if bleeding badly a band-aid.
Some OTC 'anti-scarring creams' and even simple A&D ointment can help superficially but not so much with the problems you describe of your actual veins. But your basic problem is probably indeed damaged valves or portions of the veins that have become slightly distended. This is something that will heal itself with time but obviously not if you continue with the same IV use. The in-and-out and digging for veins is the worst thing in terms of what is happening to you without a doubt. It is advisable to do this only a minimum amount and then get a new needle and try again (this becomes very difficult when there is already blood in the syringe, to tell whether you have registered, which in turn leads to missed/half-missed/subcutaneous-and-absorbing-into-the-vein shots/etc.)
Also, what size needles are you using? You may benefit from using smaller ones although there are also disadvantages to those (harder to register, can get bent easily.) I favored a 27g which is large and it did do damage to my veins; 28, 29 are common choices, 30, 31 are quite small, and, if your ability to hit your veins is good, will do less damage, but if you have to dig a lot, may become bent out of shape and do considerably more damage than a longer needle in the first places as they can form a sort of bent-wire-hanger or even hook type form when being removed. Needle size is to some degree a matter of personal preference and comfort but certainly involves tradeoffs.
